THE DIBRANCHIATA. 465 



The genus Acanthoteuthis * {Belemnoteuthis^ Pearce) — 

 one of the BeUmnitidce^ 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 JBelemnitidcB abound from the 

 Lias to the end of the Mesozoic period, after ^vhich they 

 disappear. The Sepiadm first appear in the latter half of 

 the Mesozoic epoch ; while the 2'eiithidcB are represented by 

 genera closely allied to existing forms {Teuthopsis^ Belein- 

 nosepia) as early as the Lias. 



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

 1844.) 



