PROSOBRANCHIATA. 



76l 



In a great many forms, on the other hand, siphons are not devel- 

 oped, and the mouth of the shell is simply rounded or "entire" 

 (fig. 641), when the shell is said to be "holostomatous." 



The great majority of the Prosobranchiata are inhabitants of the sea, 

 but certain groups are restricted to fresh or brackish waters ; while 

 the Cydostomidce, Aciculidce, and Helicinidce are terrestrial in habit. As 

 regards their distribution in time, the palseontological record is fuller 

 in the case of the Prosobranchiates than it is in the case of any 

 other division of the Gastropods. The Palaeozoic types of the 

 Prosobranchiates belong almost exclusively to forms in which the 

 shell is " holostomatous." On the other hand, forms with the 

 " siphonostomatous " type of shell-mouth do not make an undoubted 



Fig. 641. — Scalaria Green- 

 landica, a holostomatous 

 Univalve. 



Fig. 642. — Oliva por- 

 phyria, a siphonosto- 

 matous Univalve. 



Fig. 643. — Cerithuim 

 aluco, showing an an- 

 terior and posterior 

 notch for the siphons. 



appearance till the Trias is reached, and they become the predomi- 

 nant group of Prosobranchiates in the Tertiary rocks, which position 

 they still hold. 



The possession of a " holostomatous " or " siphonostomatous " 

 shell has been employed as the basis for a separation of the Proso- 

 branchiata into two primary sections — viz., the Holostomata and 

 Siphonostomata — but these names, though convenient as general 

 terms, do not indicate natural divisions. In the more modern and 

 more scientific classification now generally in use among Malacolo- 

 gists the characters of the " radula " or lingual ribbon are those 

 mainly relied upon as distinguishing the primary groups of Proso- 

 branchiates. The form of the "radula" is, however, necessarily 

 unknown in the case of fossil forms, and it is therefore only by 

 means of analogies — which may or may not be trustworthy — that 

 the extinct groups of Prosobranchiates can be ranged in a series 



