771 



MONOGRAPH OF THE GENUS 



PETRICOLA. 



Fam. Yeneridge. 



Char. Gen. Testa Bequivalvis, inaequilateralis, oblonga, plerumque 

 irregularis; impressionibus muscularibus duabus; pallii impres- 

 sione siuu elongate; dentibus cardinalibus duobus arcuatis pro- 

 ductisj compressis ; ligamento externo^ brevi. 



It is not easy, with any degree of consistency, to distinguish 

 the species of Petricola from those of Veneriijois. As a rule, the 

 Fetricolce inchne to taper off posteriorly, while they are round and 

 full in front ; generally their radiating ribs are more conspicuous 

 than the concentric striae or laminae; whereas in Venervpis the 

 laminae are most prominent, particularly at the back of the shell. 

 On the whole the species of the two genera, as here placed, catch 

 the eye as forming two tolerably distinct groups. 



The most doubtful species are P. elliptica and P. ml'ida, wluch 

 I have inadvertently admitted in the plates of both genera. In 

 general appearance they incline most to the genus Petricola. 



1. Petricola pholadifoemis (pi. clxvi. f. 1), Lamarck, An. 

 s. vert. V. 505, no. 11. 



Pet. testa oblonga Candida cylindracea, antice brevissima, costis 

 validis subquadratis, crebre serratis, distantibus radiata, postice 

 attenuata, costis obliquis parvis, distantibus, versus extremitatem 

 subobsoletis. 



Oblong, cylindrical, very like a Pholas, with very thick, distinct, 

 serrated ribs in front, which become smaller towards the hinder 

 part of the shell, where they sometimes become almost obliterated. 



North America. 



2. Petricola serrata (pi. clxvi. f. 11), Beshanes, Pro. Zool. 

 Soc. 1853. 



5 H 



