MOLLUSCA> 83 
in one valve, consisting of one hollowed tooth, and in the 
other a depression between two eminences. It includes 
two shells from the cabinet of Favanne. _ It is intermediate 
between the mactrae and myze. 
9. Donax. The shells of this genus are readily known 
at first sight by their singular cuneiform shape. The hinge 
teeth are two in number, and the lateral teeth are spread- 
ing. The ligament is external, and, like the tellinze, it is 
placed on the shortest side. This isa circumstance of rare 
occurrence among the inequivalve testacea. The animal 
belongs to the genus peroneea of Poli. 
Lamark has instituted a genus nearly allied to the pre- 
ceding, termed Prerricota, the shells of which gape a little 
at both ends. There is one hinge tooth in one valve, and 
a bifid one in the other. The ligament is external; the 
muscular impressions are two in number ; the structure of 
the hinge teeth, and the absence of the lateral teeth, at once 
distinguish this genus from the donax and venus. These 
animals are likewise peculiar in their habits. Lamark quotes 
the venus lithophaga of Retzius, Act. Acad. Tour. vol. iii. 
p- 11, and the venus lapicida of Chemn. Conch. x. p. 356. 
tab. 172, f. 1664-5. But Lamark’s genus has been again 
altered by Fleurieu-Bellevue, who has formed his genus 
Ruperts, from the V. lithophagus of Retzius, and an- 
other species termed striata. (Mem. de 0 Acad. de la Ro- 
chelle, ii. tab. ii, fig. 9.) In this genus the shell is transverse 
and inequilateral, compressed in the anterior part, and 
swollen behind. There are two crooked hinge teeth on 
each valve, one simple, the other bifid. The ligament is 
external, and there are two muscular impressions. ‘The 
donax irio belongs to this genus. The same author has 
