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CUTTLE-FISH

Recent.

ALLIES DURING SUCCESSIVE EPOCHS. Argonauta. Octopodidse. (inc. Leioglossa). tiTerarv, Cretaceous.

Palseoctopus.

OCTOPODA. s Baculites. Hamites. Hoplites. Hoplites.

Jurassic.

Lytoceras.

Acanthoceras. Pachydiseus. Phylloceras.

Schloenbachia. Aspidoceras.

Crioceras. Cardioceras. Perisphinctes. Cosmoceras. Heeticoceras. Ochetoceras. Stephanoceras. Harpooeratidse. Amaltbeidse. Phylloceras. Schlotheimia. Arietidse. /Egoceras.

Lytocebatida. Cosmoceratida.

AIgoceratida. Phylloceratida.

Arietida.

AN GUSTISELL ATI. Trachyceratidse. Balatonitidae.

Ceratitidse.

Monophyllites. Arcestidae. Pinacoceras. Sageceras. LEIOSTRACA.

Triassic.

TBACHY OSTRAC A. Dinarites.

boniferous. and Car- Devonian. Permian

Prolecanites.

Neolobites. Placenticeras. Tissotia.

Tirolites. Pericyclus.

“ Goniatitidse.”

Popanoceras.

Cyclolobus.

Medlicottia.

LATISELLATI. Olymenia.

Prolecanites. AMMONOIDEA. SOSI PROTOCONCH I A.

rian. SiluOrdovician. brian. Cambeing Haplosiphonida. This condition of things continued to the Carboniferous period, after which no complex siphuncles are known; the organ then became insignificant, and is finally lost in all the recent Cuttle-fish except the Spirula. The position of the siphuncle was variable at first, and in those forms which are the special ancestors of the Nautilus it continued to be so. But in Devonian times a cleavage of the class took place, and a new group was developed, in which the siphuncle is permanently pushed to the ventral side in the adult, where it is also now found in the archaic Spirula. This group is also characterized by having a thin and fragile shell, and by the peculiarities of the initial chamber or protoconch, which is of globular form, has a restricted neck, and contains the commencement of the siphuncle at its centre, as seen in Spirula. We thus obtain two great subdivisions of the Class: on the one hand the Nautiloidea, presumably tetrabranchiate and

certainly less progressive; on the other, the remaining Cephalopoda then existing. The only collective name yet suggested for this new group, unless we assume them all to have belonged to the Dibranchiata, is the Sosiprotoconchia of Bather, who calls the Nautiloids at the same time Lipoprotoconchia. As the Cuttle-fish proper must have descended from these Sosiprotoconchia, we need follow the Nautiloids no more. Within the Sosiprotoconchia there was soon a new and fundamental cleavage dependent on the variation of the coiling. On the one side are the Belemnoids, which have practically no coiling at all, and whose fragile shells are protected, if at all, by a massive external guard, while their septa remain simple; on the other side are the Ammonoids, whose globular protoconch is closely enveloped by the later coils, which often overlap each other and whose septa become complex, their surface being partially convex forwards, and their edges or “ sutures ” folded or wrinkled. S. III.— 41