MODIOLARIA. 121 



portion smooth : heals incurved : hinge mostly toothless, but 

 sometimes crenulated : hinge-plate finely notched. 



This genus differs from Mytilus in the mantle being 

 folded into a distinct tube for the supply of food and 

 aerated water, as well as in the remarkable sculpture of 

 the shell. The hinge-plate is crenulated as in Mytilus 

 phaseolimts. The foot is very flexible and extensile ; and 

 when fully stretched out, it is two or three times as long 

 as the shell, and becomes almost thread-shaped. The 

 animal can crawl rather fast along a level surface by 

 extending and fixing the point of its foot in front, using 

 it as a fulcrum, and then dragging itself forwards ; and 

 it occasionally floats on its back like Kellia suborbicu- 

 laris and other small bivalves. One species (M. mar- 

 morata) burrows into the tunic or outer coat of Asci- 

 dians, and others form nests or wrapping-cases with the 

 aid of their byssus. It would seem, however, that the 

 European habit of infesting Tunicata is not possessed by 

 any Transatlantic species. 



Although Beck has the credit of founding the genus 

 Modiolaria, and it has been adopted by Loven, Mid- 

 dendorfF, and other writers on the northern mollusca, 

 it has never been described or characterized by any of 

 them. The same remark applies to the genera La- 

 nistes of Humphreys and Lanistina of Gray, both of 

 which are synonyms of the present genus. Modiolaria 

 may be distinguished from Brown's genus Crenella by 

 the shell of the latter being covered all over with strise 

 or ribs, and by its having a strong and crenulated 

 tooth in the hinge of each valve. The animal also 

 differs in some important particulars, which will be pre- 

 sently noticed. Whether all these differences are gene- 

 ric or subgeneric, may be a moot question ; but as the 

 lines of demarcation between any one group of animals 



G 



