PETRICOLA. 



Lam. Hist. Nat. des Anim. sans vert. V. 503. 



TESTA sequivalvis, insequilatera, transversa, 

 subtrigona, vel oblonga, latere postico rotun- 

 dato, antico plus minus ve attenuate, plerumque 

 pauliim hiante. Cardo^ dentibus duobus in 

 utraque valva, interdum acutis, recurvis ; inter^ 

 dum brevibus, obtusis. Impressiones musculares 

 duae, antica plerumque rotundata, postiea ob- 

 longa. Sinus impressionis pallii adhaerentis 

 museuli magnus. Ligamentum externum. 



The Genus Petricola, as it stands at present, is composed 

 of several shells which Lamarck formerly thought sufl&ci- 

 ently different to form two genera, his Petricola and 

 Rupellaria, the first with two cardinal teeth in one valve 

 and one in the other, the second with two teeth in each 

 valve ; but we entirely agree with him in the propriety of 

 uniting them, not only because they appear to he subject 

 to variation in the number of teeth, but because the 

 specimens existing in cabinets are very seldom perfect, 

 and do not consequently exhibit the same number of teeth 

 as exist in nature. But we certainly cannot profess to be 

 so well satisfied with the place Lamarck has assigned to 

 this and some other genera which form the hollows 

 in stone in which they dwell; we think that a great degree 

 of similarity, in external figure and appearance as well as 

 habit, should have brought them nearer to the Pholadarice. 



