PETRICOLA. 



Shell eqiiivalve, inequilateral, transverse, generally 

 rather triangular, hut sometimes transversely elongated 

 and even suhquadrate; the posterior side rounded; the 

 anterior side also rounded, generally more or less at- 

 tenuated, and for the most part rather gaping. Two 

 cardinal teeth in each valve, which are sometimes acute 

 and recurved, particularly the posterior tooth in one 

 valve, and the anterior in the other; sometimes also the 

 teeth are internally grooved, and the anterior one in one 

 valve is hroad and hifid; sometimes the teeth are short 

 and obtuse. The muscular impressions are two ; that on 

 the anterior side suborhicular, the posterior rather oblong, 

 and there is a large sinus in the impression of the muscle 

 by which the mantle adheres to the shell. The ligament 

 is external, but it is sometimes almost hidden by the pro- 

 minent anterior edge of the valves near the umbones. 



The Petricolae are marine; they live in cavities, 

 evidently of their own working, though on account of their 

 form they cannot possibly have been produced by a rotatory 

 motion, for they are exactly of the shape of the shell 

 Uself, and a very^little larger. Several species have, the 

 same form and a^e externally rough lil^e the Phplfides, 

 others are ndore irregular and approach nearer to the 

 Saxicavce and Venerirupes; to what Linnean Genus they 

 belong it is difficult to say: such species as have been 

 described by Linneans have been placed in Venus and 

 Donax, We have represented the following species in 

 our plate, two of which being hitherto undescribed, we 

 have added the specific characters : 



Fig. 1. Petricola pholadiformis, Lam. Outside. 



2. — — ^ — — : ^ — Inside of both valves. 



3. — Dactyltts, Nob. Tpst^ transversim oblongJi subgibb^; la- 

 tere postico breviore^ sulcis longitudinalibus, striis incrementi decussatis, radiate ; 

 antico subglabro. . 



4. ■ > ochroleuca, Lam. 



5. — rupestris. Vemis rupestris, Broccbi. 



6. subglobosa, Nob. Testa subglobos^, latere postico rotim- 



dato, antico subtruncato; costulis longitudinalibus, decussatis, aatice distan- 

 tioribus, altioribus. ■ : < 



