THE DIBRANCHIATA. 465 



The genus Acanthoteuthis * {JBelemnotenthis^ Pearce) — 

 one of the JBelemnitidoe, 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 JBelemnitidce abound from the 

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

 disappear. The Sepiadm first appear in the latter half of 

 the Mesozoic epoch ; while the Teuthidm are represented by 

 genera closely allied to existing forms {Teiithopsis^ JBelern- 

 nosepia) as early as the Lias. 



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



18M.) 



