MOLLUSCA: CEPHALOPODA. 357 



complex in structure, and was calcareous throughout its entire 

 length. 



The structure of the shell in the Ammonitidce is exactly that 

 of the Pearly Nautilus, consisting of an outer porcellanous 

 and an inner nacreous layer. The body-chamber was rather 

 elongated than laterally expanded or dilated. The simplest 

 form of the Ammonitidce is the Baculite, in which the shell is 

 straight, like that of an Orthoceras, whilst the septa have the 

 characters of those of an Ammonite, and the siphuncle is ex- 

 ternal. In the Turrilite the structure of the shell is the same, 

 but it is coiled into a turreted spiral. In the Ammonite itself, 

 the shell is discoidal and involuted, corresponding (mform) to 

 the shell of the Nautilus ; the body-chamber was of compara- 

 tively large size, and had its aperture closed, in some species 

 at any rate, by an operculum. The shell sometimes attained 

 a gigantic size, and several hundred species of the genus have 

 been described. In Crioceras the shell was a fiat spiral, like 

 that of the Ammonites, but the whorls are not in contact. In 

 Toxoceras the shell is shaped like a bow. In Ancyloceras the 

 shell is at first discoidal, with separate whorls, then produced 

 into a straight line, and finally bent forwards into a hook. 



SYNOPSIS OF THE FAMILIES OF THE CEPHALOPODA. 

 CLASS CEPHALOPODA. 



ORDER I. DIBRANCHIATA. 



Animal with two branchiae ; not more than eight or ten 



arms,*provided with suckers ; an ink-bag ; shell commonly 



internal and rudimentary ; rarely external, but not cham- 

 bered. 



SECTION A. OCTOPODA. 



Arms eight, suckers sessile. 

 Fam. i. Argonautid<z. 



Female provided with a calcareous, external, monothalamous 

 shell, secreted by the webbed extremities of the dorsal arms. 

 Gen. Argonauta. 

 Fam. 2. OctopodidcB. 



Shell internal, rudimentary, uncalcified. No pallial fins in 

 most. 111. Gen. Octopus, Tremoctopus, Eledone, Pinnoctopus. 



SECTION B. DECAPODA. 



Arms eight, with two clavate "tentacles;" suckers pedun- 

 culated. 

 Fam. 3. Teuthida. 



Shell an internal horny "pen" or "gladius." Fins mostly 

 terminal. 111. Gen. Loligo, Onychoteuthis, Ommastrephes. 

 Fam. 4. Belemnitidce. 



Shell internal, composed of a conical chambered portion 

 ("phragmacone") with a marginal siphuncle, sometimes pro- 



