THE CEPHALOPODA 



337 



slight development of the pro-ostracum, and the large size of the 

 phragmocone. The group is probably derived from the straight 

 Tetrabranchia, such as Bactrites. In the living genera, with the 

 exception of Spirula, the shell is a chitinous gladius. 



FAMILY 1. BELKMNOTEUTHIDAE, Zittel. An extinct family in which 

 the shell has a well -developed phragmocone and the rostrum is reduced 

 to a calcareous envelope surrounding it ; the siphuncular necks are 

 directed backwards, as in the Nautiloidea ; there were ten equal arms 

 provided with hooks. Genera Phragmoteuthis, Mqjsisovics ; Trias. 

 BelemnoteuthiSj Pearce ; Jurassic and Cretaceous (Fig. 262, A). Acantho- 

 teuthis, Wagner and Minister ; Jurassic. FAMILY 2. ADLACOCERATIDAE, 

 Fischer. An extinct family in which the shell is formed of a phragmocone 



Fio. 295. 



Spirula. A, dorsal aspect ; B, ventral aspect, a, arms, e, eyes ; fi, fins ; fu, funnel ; pa, 

 mantle ; po, posterior fossa ; sh, shell ; te, tentacular arms ; t.d, terminal pallial disc. (After 

 Chun.) 



with widely distant septa ; the siphuncular necks are directed anteriorly ; 

 the rostrum is well developed and claviform. Genera Aulacoceras^ Hauer ; 

 Trias. Atractites, Giimbel ; Trias and Jurassic. Xiphoteuthis, Huxley ; 

 Lias. FAMILY 3. BELEMNITIDAE, de Blainville. An extinct family 

 with a short phragmocone provided with a ventral siphuncle and 

 prolonged dorsally into a long pro-ostracum ; the rostrum highly 

 developed and cylindrical. Genera Bekmnites, Lister ; 350 species 

 from the Jurassic and Cretaceous. Diploconus, Zittel ; Upper Jurassic. 

 FAMILY 4. BELOPTERIDAE. Rostrum and phragmocone well developed ; 

 the phragmocone often curved ; the initial chamber small. Genera 

 Beloptera, de Blainville;. Eocene. Bayanoteuthis, Munier-Chalmas ; 

 Eocene. Spirulirostra, d'Orbigny ; Miocene (Fig. 262, C). FAMILY 5. 

 SPIRULIDAE, d'Orbigny. The two dorsal and ventral sides of the aboral 



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