PETRICOLA. 153 



a striking characteristic. A narrow strip near the hinder 

 dorsal edge is often free from the radiation, and here the 

 concentric wrinkles are usually the most evident. The 

 ventral edge is much arcuated, ascending at both extre- 

 mities, but more suddenly so anteriorly. The front dorsal 

 edge is elongated, and not greatly declining, its outline 

 has some disposition to convexity after quitting the 

 ligament which appears to be dark, of modei-ate size, and 

 rather prominent, but an examination of more specimens is 

 required before this can be asserted with certainty. The 

 (strictly limited) front dorsal edge is extremely short, and 

 scarcely sloping, but it almost forms one sweep with the 

 arcuated anterior margin. The front side of the shell is 

 broad and very short ; its extremity is rounded, but not 

 symmetrically so, owing to the upper portion not curving 

 equally with the lower. The produced posterior side be- 

 comes attenuated towards its extremity, which is more 

 or less bluntly rounded. In the specimen figured this 

 is more attenuated and elongated than is usual in the 

 species, and the elevated radiating lines are coarser than in 

 our Mediterranean exam])les. The umbones are tumid 

 and rather prominent, and the beaks are small, much 

 inflected, and scarcely leaning to either side. There is 

 neither lunule nor umbonal ridge. The hinge consists of 

 two primary teeth in each valve, which are small, narrow, 

 much elevated, projecting inward, and very caducous : 

 the larger of these two, which is the anterior in the 

 left, and the posterior in the right valve, is deeply cloven 

 at its apex, where it is broader than at its base, which 

 sulcus is continued below, even to the hinge margin ; the 

 smaller tooth is less raised, simple, narrow, and more 

 oblique. The inner margin is not crenated ; the palleal 

 sinus is ample and rounded. 



VOL. I. X 



