INVERTEBRATES. 335 



sinuosity. Anterior ear rather larger than the other, but not 

 equaling the prominence of the margin below; in the right 

 valve rounded at the extremity, and defined by a deep, sharply 

 cut byssal sinus, from the extremity of which a small sulcus 

 extends obliquely upwards to the anterior side of the beak ; in 

 the right valve flattened, broader, and truncated so as to meet 

 the hinge line at an obtuse angle, with but a very slight mar- 

 ginal sinuosity, rather distinct from the oblique anterior umbo- 

 nal slope. Beaks small, rather compressed, nearly or quite 

 equal, and terminating a little behind the middle of the cardi- 

 nal margin ; anterior umbonal slopes oblique, and more distinct 

 in both valves from the ear, as well as more produced, than 

 the posterior. Surface, to the unassisted eye, apparently 

 smooth, but when examined with a magnifier, seen to be 

 marked with very fine, regular, closely arranged concentric 

 striae, which, on the anterior ear of the right valve, become 

 suddenly much coarser and more strongly defined. In some 

 instances, the faintest possible traces of something like radia- 

 ting ribs may be seen on the right valve, in a cross light, under 

 the magnifier. This latter character, however, is rarely visi- 

 ble, and would scarcely ever be observed unless carefully looked 

 for. Diameter, from hinge to pallial margin, 0.57 inch; antero- 

 posterior diameter, 0.53 inch; convexity, 0.10'inch; length of 

 hinge line, 0.27 inch. 



This little shell is evidently closely allied to Pecten "pusillus, Schlotheim, from 

 the European Permian rocks. Compared with Prof. King's figures of that 

 species (Permian Possils of England, pi. xiii, figs. 1, 2, 3 and 4), it differs in 

 having no radiating eostae visible to the unassisted eye, while its anterior out- 

 line does not, in the right valve, round so regularly into the byssal notch above. 

 Prof. King's figures of the left valve, which he says are too oblique, differ from 

 that of our shell in having the obliquity in the opposite direction. On com- 

 paring Prof. McCoy's description of P. pusillus (Brit. Pal. Foss., p. 477) with 

 our shell, we have been struck with its almost exact agreement, and led to sus- 

 pect that they may prove identical. 



Dr. Geinitz has also figured, in his beautiful work on the Permian Fossils of 

 Germany (Dyas, pi. xv and xix), under the names Pecten pusillus and P. sert- 



