434 VENERID^. 



branchial with its orifice fringed, the anal simple. Foot 

 lanceolate. Labial palps small, triangular. 



We have constituted this genus for the reception of the 

 Venus undata of authors, a shell which has been now placed 

 in one group, now in another, as the inclinations of the 

 systematist may have directed, but always with a doubt 

 respecting its true position. Presenting the external ap- 

 pearance of a Lucina^ whilst its internal structure linked it 

 more closely with Venus and its allies, it had become a 

 sort of conchological puzzle, and yet, though many a shell 

 with fewer claims to distinction had been elevated to the 

 rank of a generic type, distrust respecting its true nature 

 seems to have prevented this one enjoying such an honour. 

 The deep pallial sinus should have sheAvn those who asso- 

 ciated it with the Lucinida that it had no jilace in that 

 family. Its evident affinities were with the Venus tribe, 

 but from Venus proper and equally from Gytlierea and Pul~ 

 lastra marked features of dentition or margin, independent 

 of peculiarity of habit, separated it. With Artemis it had 

 more connection, chiefly, however, in external aspect, for its 

 hinge and pallial sinus are very different. The discovery of 

 the animal has shewn that its peculiarities indicate essential 

 differences in the organization also, and that the characters 

 of its foot, mantle, and siphon, are such as to place its 

 claim to generic rank beyond a question. It seems to have 

 considerable affinity with the fossil genus Thetis^ in which, 

 however, the pallial sinus is lanceolate and prolonged al- 

 most to the beaks, whilst the external surface is curiously 

 punctated. 



