MO'LLUSCA. 471 



1905. Pecten burlingtonensis Johns., Proc. Acad. Nat. Sci. 



Phil. (1905), p. II. 

 1905. Pecten perlamellosus Johns., Proc. Acad. Nat. Sci. Phjl. 



(1905), p. 12. 



Description. — Shell, in large individuals, attaining a height of 

 57 mrh., and a width of 62 mm. ; the hinge-line straight, one- 

 half or a little less than one-half the width of the shell, with a 

 central triangular cartilage pit; the body of the shell broadly 

 subovate in outline, the auriculations moderately large and nearly 

 equal in size ; the cardinal slopes a little concave, diverging from 

 the beak at an angle of 90° or more, the shoulders of the valves 

 prominent and above the middle of the height of the shell. Left 

 valve depressed convex with the auriculations sharply differen- 

 tiated. Right valve nearly flat, with a moderately deep byssal 

 sinus. Surface of the valves marked by concentric bands which 

 are continuous across the auriculations, and by exceedingly fine, 

 impressed, radiating stride which are continuous upon the auricu- ' 

 lations and the umbo, where they are about equal in width with 

 the interspaces, but on the outer portion of the shell they become 

 more or less discontinuous, the inner portion of the concentric 

 bands often being nearly smooth, while on the outer portion they 

 are completely striate, but with the interspaces between the striae 

 broader than the strias themselves. 



Remarks. — -Whitfield has united Pecten bellisculptus Con., with 

 Pecten burlingtonensis 'Gabb, but an examination of a large num- 

 ber of individuals in the recent collections O'f the Survey show the 

 two species to be entirely distinct. The two forms have some- 

 times been considered as the opposite valves of the same species, 

 but this cannot be so for one example of P burlingtonensis has 

 been examined which preserves the cast of both valves, and they 

 do not essentially differ in the character of the markings although 

 the right valve is much flatter than the left. In the present report 

 P. bellisculptus is considered as synonymous with P. argillensis. 



The surface markings, even in internal casts, are usually suffi- 

 cient to separate the two forms, the conspicuous feature of bur- 

 lingtonensis being the concentric bands, the radiating striae usually 

 being entirely wanting in the casts, while in argillensis the 



