PLAGIOSTOMA. 



Lluyd, 



TESTA inaequilateralis, subaequivalvis, subaurita 

 altitudine longitudinem plerumque superante ; 

 sulcis vel striis ab umbone ad marginep- ven* 

 tralem deeurrentibus ; linea cardinali in altera 

 valva rectiusciila, in altera plus minusve angu* 

 losa; valvis clausis aperturam trigonam ear- 

 dinalem elForniante : iimbones remotiuseuli: 

 cardo edentulus. 



A remarkable Genus of Bivalves only known in a fossil 

 state ; it is nearly related to Lima, to which indeed some 

 Naturalists would unite it. We have already stated our 

 reason for thinking it distinct ; although we are decidedly 

 of opinion that it must be arranged close to it in the 

 family of the Pectinidee. This Genus which was estab- 

 lished by Lluyd who named it,* from the singular appear- 

 ance of the hinge when seen in a particular direction, is 

 of some importance to Geologists, inasmuch as the various 

 species are characteristic of the various beds of Limestone 

 from the Carboniferous Limestone up to the Chalk, abound- 

 ing in most of the intermediate oolitic beds ; the typical 

 species named giganteum being found in great profusion 

 in the Lias, and the spinosrim being almost as frequent in 

 the upper Chalk. The descriptions and figures of many 

 species may be consulted in Sowerby's Mineral Concho* 

 logy of Great Britain, where the localities and the bed to 

 which each is peculiar are pointed out. 



Plagiostoma signifies " mouth awry." 



