THE DIBRANCHIATA. 465 



The genus Acanthoteuthis l (Belemnotenthis, Pearce) — 

 one of the Belemnitidoe, 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 Belemnitidm abound from the 

 Lias to the end of the Mesozoic period, after w T hich they 

 disappear. The Sepiadw first appear in the latter half of 

 the Mesozoic epoch ; while the Teuthidoe are represented by 

 genera closely allied to existing forms (Teuthopsis, .Belem- 

 nosepia) as early as the Lias. 



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

 1844.) 



