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7. Dentati

8. Gemmati.

9. Flexuosi

10. Compressi II. Armati

12. Angulicostati

13. Capricorni
14. Heterophylli
15. Ligati.
16. Planulati.

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17. Coronati
18. Macrocephali.
19. Globosi

20. Fimbriati

Oolitic and Cretaceous.-Ex. A. Jason.
Trias.-Ex. A. Aon.

SECTION E. Back squared.

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SECTION F. Back round.

Lias.-Ex. A. planicostatus.
Oolitic.-Ex. A. heterophyllus.
Cretaceous.-Ex. A. Mayorianus.
Oolitic.-Ex. A. annulatus.

Oolitic.-Ex. A. Humphresianus.

Oolitic and Cretaceous.-Ex. A. microstoma.
Trias.-Ex. A. globus.

Jurassic and Cretaceous.-Ex. A. sub-fimbriatus.

In the genus Crioceras are included forms which resemble the Ammonites in all essential characters, but in which the volutions of the shell are not contiguous. The shell, therefore, is discoidal, with separate whorls, thus corresponding with Gyroceras amongst the series of the Nautilida. All the known species of Crioceras belong to the Cretaceous period, ranging from the Lower Greensand to the Gault.

In the genus Toxoceras the shell is simply arcuate, or bent like a horn, and is never spirally rolled up; so that this genus represents Cyrtoceras in the series of the Nautilide. The species of Toxoceras range from the lower Oolites to the Gault, but the genus is characteristically Cretaceous.

In the genus Ancyloceras (fig. 262) the shell at first re

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Fig. 262.-Ancyloceras Matheronianus. Gault.

sembles that of Crioceras, consisting of several volutions which are coiled into a flat spiral, but which are not in contact with one another. The shell differs from Crioceras, however, in the fact that the last volution is produced at a tangent, and is ulti

mately bent back in the form of a crosier. The species of Ancyloceras are Jurassic and Cretaceous, ranging from the Inferior Oolite to the Chalk.

In the genus Scaphites, the shell resembles that of. Ancylo ceras in consisting of a series of volutions coiled into a flat

Fig. 263.-Turrilites catenatus. Gault.

spiral, and having the last volution detached from the others, produced, and ultimately bent back in the form of a crosier. Scaphites differs from Ancyloceras in the fact that the volutions of the enrolled part of the shell are in contact, instead of being separate as they are in the latter. The produced whorl, also, is rarely of any great length, but is speedily bent back upon itself. All the species of Scaphites are Cretaceous, ranging from the Lower Greensand to the Chalk.

In the genus Helicoceras the shell is coiled into a turreted spiral, the volutions of which are not contiguous. The shell is, also, left-handed or "sinistral." With the exception of a single species from the Inferior Oolite, all the species of Helicoceras belong to the Cretaceous period.

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66

In the genus Turrilites the shell agrees with that of the preceding in being composed of volutions which pass obliquely round a central axis (fig. 263), so as to form a turreted spiral. The shell is, also, left-handed or sinistral." In Turrilites, however, the whorls of the shell are in contact, instead of being disconnected as they are in Helicoceras. The genus corresponds with Trochoceras in the series of the Nautilida. All the species of Turrilites are Cretaceous, ranging from the Gault to the Chalk.

In the genus Hamites the shell is an extremely-elongated cone, which is bent upon itself more than once, in a hook-like manner, all the volutions being separate. Numerous species of Hamites are known, all of them being Cretaceous, and ranging from the Lower Greensand to the Chalk,

In the genus Ptychoceras the shell is, also, a much elongated cone, which is simply bent upon itself once, the two straight portions of the shell being in contact. The range of this genus is the same as that of Hamites, extending from the Lower Greensand to the Chalk.

Lastly, in the genus Baculites the shell is simply a straight elongated cone, not bent in any way. Baculites corresponds, therefore, with Orthoceras in the series of the Nautilida. The range of Baculites is the same as that of the preceding--from the Lower Greensand to the Chalk; but the genus is most abundant in the Chalk itself.

CHAPTER XXVI.

DIBRANCHIATE CEPHALOPODS.

THE Dibranchiate Cephalopods or Cuttle-fishes are characterised as being swimming animals, almost invariably naked, with never more than eight or ten arms, which are always provided with suckers. There are two branchia, which are furnished with branchial hearts; an ink-sac is always present; the funnel is a complete tube, and the shell is internal, or, if external, is not chambered.

The Cuttle-fishes are rapacious and active animals, swimming freely by means of the jet of water expelled from the funnel. The arms constitute powerful offensive weapons, being excessively tenacious in their hold, and being sometimes provided with a sharp claw in the centre of each sucker. They are mostly nocturnal or crepuscular animals, and they sometimes attain to a great size. They may be divided into two sections, Octopoda and Decapoda, according as they have simply eight arms, or eight arms and two additional "tentacles."

The parts of a Dibranchiate Cephalopod which may be preserved in a fossil condition are the mandibles, the ink-sac, the shell (if such be present), and the internal skeleton. The occurrence of the mandibles and ink-sacs of Dibranchiate Cephalopods in a fossil state has been already spoken of (p. 273), and need not be further noticed here. An external shell is present only in the Argonaut amongst living Cuttle-fishes, and similar structures are of rare occurrence as fossils in some of the youngest portions of the earth's crust. The internal skeleton of the Cuttle-fishes differs very much in its characters

in different cases. In the Calamaries the skeleton is in the form of a horny "pen," consisting of a median shaft and of two lateral expansions or wings. In the Sepiada the skeleton has the form of a broad, laminated, calcareous plate, having a more or less perfectly chambered apex or "mucro." In the singular Spirula the skeleton has the form of a chambered tube coiled into a spiral, the coils of which are separate from one another. Lastly, in the extinct family of the Belemnitida, there was a complicated internal support. It is, then, chiefly from the preservation of their internal skeletons that the Dibranchiate Cephalopods are known to have existed in past periods of the earth's history. In addition, however, to the skeleton, mandibles, and ink-bag, cases are not altogether unknown in which the hooks of the suckers, and even the outlines of the arms and body, have been preserved in a fossil condition.

As regards their general distribution in time, the record of the Dibranchiate Cephalopods is much less complete than that of the Tetrabranchiata. In the vast series of the Palæozoic formations no trace has ever been discovered of the existence of any member of this order. Shortly after the commencement of the Mesozoic period appear the first Belemnites; and all the Secondary formations after the oldest teem with the remains of this family of the Dibranchiata. Remains of the living families of the Teuthida and Sepiada are also not unknown in the Mesozoic Rocks, but no trace of the great group of the Belemnitida has hitherto been detected in Tertiary deposits. Upon the whole, the order must be regarded as having attained its maximum at the present day. In the following are given the characters, chief genera, and distribution in time of the families of the Dibranchiate Cephalopods.

SECTION A. OCTOPODA.-The Cephalopods comprised in this section are distinguished by the possession of eight arms, which are provided with sessile suckers. The body is short and bursiform, ordinarily without fins. The shell is internal and rudimentary; in one instance only (Argonaut) external.

FAM. I. ARGONAUTIDE.-Female provided with a delicate, symmetrical, involuted shell, which is secreted by the webbed extremities of the two dorsal arms, and is not attached in any way to the body of the animal. Male much smaller than the female, shell-less. This family includes only the single genus Argonauta (the Paper Nautilus). One or two species of Argonaut have been discovered in the Pliocene Tertiary.

FAM. 2. OCTOPODIDE.-Shell internal, rudimentary, represented by two short styles encysted in the substance of the

mantle. This family includes the living Poulpes and their allies, but has no fossil representatives.

SECTION B. DECAPODA.-The Cuttle-fishes of this section have eight "arms" and two additional "tentacles," which are much longer than the true arms, and carry suckers on their extremities only, which are expanded and club-shaped. The suckers are pedunculated, the body is furnished with lateral fins, and the shell is always internal.

FAM. 3. TEUTHIDAE.-Shell consisting of an internal horny "pen" or or "gladius," composed of a central shaft and two lateral wings. Several of such pens may exist in a single individual, packed one behind the other. Fins mostly terminal and angular. This family comprises the living Calamaries and Squids, and the following fossil genera have been founded upon "pens" which have been discovered in various Secondary deposits.

a. Teudopsis.-Pen lanceolate, produced in front, dilated and spatulate behind. Five species of this genus have been described from the Lias.

b. Beloteuthis.-Pen lanceolate, pointed in front, with two small wing-like expansions behind (fig. 264). Six species have been described by Count Münster from the Upper Lias of Würtemberg.

c. Leptoteuthis.-Pen horny, hastate, broad in front, pointed behind. A single species is known from the Oxford Clay (Jurassic).

d. Besides the above, remains found. in the Jurassic Rocks have been referred to the living genera Enoploteuthis and Ommastrephes; and the extinct genus Acanthoteuthis has also been placed in this family.

FAM. 4. SEPIADE.-Internal skeleton in the form of a broad, laminated, calcareous plate, with an imperfectly-chambered. apex (or "mucro"). The chambered

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costata. Jurassic (Lias).

portion of the skeleton corresponds with Fig. 264.-Beloteuthis subthe "phragmacone" of the Belemnites.

The fossil species of this family range from the Middle Oolites upwards, and belong to the following three genera :

a. Sepia.-Shell broad and thick in front, laminated, and terminating in a prominent mucro. The fossil forms belong

to the Oxford Clay (Jurassic) and Eocene Tertiary, and the genus attains its maximum at the present day.

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