184 



CERITHIADi^i:. 



Ill this family we include Cerlthlum and Aporrhais, ge- 

 nera remarkable among canaliculated shells for the muzzle- 

 shaped heads and corresponding features of organization of 

 the animals which construct them. They seem to consti- 

 tute a group in many respects intermediate between the 

 holostomatous and Si^ihonostomatous Pectinibrcmchiata, par- 

 taking of and mingling many of the characters of both. 

 They are closely allied on the one hand to the Turritellida 

 with which family Cerithium has intimate relations, and 

 on the other to the Scalariada;, the latter relationship 

 being better seen and traced through A^iorrhals in fossil 

 than in living examples of the tribe, some fossils of the 

 last-named genus approaching very closely to Scalaria. 

 The Cerithiada serve to warn us how we trust to the shell 

 alone as a clue to natural relations, for in them we have 

 an assemblage of creatures which the collector apparently 

 not unreasonably would rather place beside the Muricida 

 than where they really should be, since in them the form 

 of the shell (pneuma-skelcton) is of but slight importance 

 compared with the modifications of the organs of their 

 respiratory and nutritive systems. The canal of the orifice 

 of the shell, indeed, depends on the presence of a rudi- 

 mentary siphonal fold, such as we see in the Littorimda, 

 and not of a prolonged siphonal process, such as the Muri- 

 cidee possess. 



