MOLLUSCA I CEPHALOPODA. 281 



body-chamber, in which the animal was lodged, appears to 

 have been comparatively small. The siphuncle is usually 

 very complex in structure, and was calcareous throughout its 

 entire length. 



Fig. S7.Orthoceras explorator, Billings. 1. Side view of a fragment, showing 

 the septa. 2. Transverse section of the same showing (s) the siphuncle. 



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 external. In the Turrilite the structure of the 

 shell is the same, but it is coiled into a spiral. In the Ammonite 

 itself, the shell is discoidal and involuted, corresponding (in 

 form) to the shell of the Nautilus ; the body-chamber was of 

 comparatively large size, and had its aperture closed, in some 

 species at any rate, by an operculum. 



SYNOPSIS OF THE FAMILIES OF THE CEPHALOPODA. 

 CLASS CEPHALOPODA. 



ORDER I. DIBRANCHIATA. 



Animal with two branchise ; not more than eight or ten arms, 

 provided with suckers; an ink-bag; shell commonly internal and 

 rudimentary ; rarely external, but not chambered. 

 SECTION A. OCTOPODA. 



Arms eight, suckers sessile. 

 Fam. 1. Argonautidce. 



Female provided with a calcareous, external, monothalamous 

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

 Gen. Argonauta. 

 Fam. 2. Octopodida. 



Snell internal, rudimentary, uncalcified. No pallial fins in 

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

 SECTION B. DECAPODA. 



Arms eight, with two clavate ' tentades ; ' suckers peduncidated. 

 Fam. 3. TeutUdce. 



Shell an internal horny 'pen' or ' gladius.' Fins mostly ter- 

 minal. 111. Gen. Loligo, Onychoteuthis, Ommastrephes. 

 Fam. 4. Belemnitida. 



Shell internal, composed of a conical chambered portion 

 (' phragmacone ') with a marginal siphuncle, sometimes produced 

 into a horny plute or ' pen,' and lodged in a cylindrical fibrous 

 'guard.' 111. Gen. Belemnites, Belemnitella, Belemniteuthis. 



