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 nayae. 



9- DONAX. The shells of this genus are readily Imown 

 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 tellinae, it is 

 placed on the shortest side. This is a circumstance of rate 

 occurrence among the inequivalve testacea. The animal 

 belongs to the genus peronaea of Poli. 



Lamark has instituted a genus nearly allied to the pre- 

 ceding, termed PETRICOL.A, the shells of which gape a little 

 at both nds. 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 

 RUPELLAHIA, from the V. lithophagus of Retzius, and an- 

 other species termed striata. (Mem. de VAcad. 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 



