THE DIBRANCHIATA. 465 



The genus Acanthoteiithis * [BeUmnoteuthis, Pearce) — 

 one of the Belemnitidce, in which the guard is almost rudi- 

 mentary, while the pro-ostracum is large and penlike — oc- 

 curs in the Trias, and is the earliest-known Dibranchiate 

 Cephalopod. The ordinary Belemnitldm abound from the 

 Lias to the end of the Mesozoic period, after which they 

 disappear. The Sepiadce first appear in the latter half of 

 the Mesozoic epoch ; while the TeuthidcB are represented by 

 genera closely allied to existing forms (7euthopsis, Belem- 

 nosepid) as early as the Lias. 



1 Owen, "A Description of Certain Belemnites," etc. ("Phil. Trans.," 

 1844.) 



