250 
ORGANIC REMAINS 
or any other country, “ British Animals,” favoured 
me with the following observations. — “Of the 
propriety of referring these shells to the genus Cydas, 
no doubt can exist. The two bifid hinge teeth, in 
some specimens, are distinctly displayed ; even the 
ligament obviously exists, and sometimes the 
ciitide. 
Cydas media This is the strong shell with 
the concentric ridges coarser, and the posterior 
side-lip more produced, than the next : the Cydas 
cornea., is, I suspect, the young of this species. 
Cydas membranacea. This is more inequilateral 
than the former, by the production of the hinder 
extremity ; it appears to have been concentrically 
oTooved.” 
O 
1. Unio antiquus. 3. Unio aduncus. 
2. Unio coinprcssns. 4. Unio porrectns. 
