33 A PALEONTOLOGY OF ILLINOIS. 



jMtsillus, Minister, as well as some of the forms generally referred to Aviculo- 

 pccten. They probably all differ from the latter genus, not only in their trun- 

 cated, or smaller posterior ear, and greater thinness, but in having no cardinal 

 area, or at least only a merely linear one. Like the typical species of the 

 latter genus, they are doubtless not provided with the cartilage pit, so charac- 

 teristic of the more modern types of the Pectinidse. 



Prof. Winchell, has proposed the name Pernopecten* {Proceed. Acad. Nat. 

 Sci. Philad., July, 1865, p. 125), for a somewhat similar type. That genus, 

 however, differs in having a central cartilage pit under the beaks, with smaller 

 pits along the hinge on each side, while it wants the deep byssal sinus under 

 the anterior ear of the right valve, seen in Streblopterm. 



Streblopteria ? TENUILINEATA, M. and W. 



PI. 26, fig. 9 a, 9 6. 



Pecten tenuilineatus, Meek and Worthen, October, 1860. Proceed. Acad. Nat. Sci., 

 Philad., p. 452. 



Compare P. pusillus, Scblot., 1816. Akad. Munch., vol. vi,p. 31, pi. vi, fig. 6a, 66, 6c. 



Shell small, compressed, thin, very nearly equivalve, broadly 

 subovate in outline, with a slight backward obliquity; ventral 

 margin regularly rounded; posterior margin forming a broad 

 gentle curve along the middle and below, and intersecting the 

 hinge above at an angle of about 120 ° ; anterior margin promi- 

 nent, rounding regularly from the termination of the produced 

 anterior umbonal slope into the base ; hinge line very short, or 

 only about equaling half the antero-posterior diameter of the 

 valves, ranging nearly at right angles to the longer axis of the 

 shell. Posterior ear of both valves nearly obsolete, obliquely 

 truncated, and compressed, but not very .distinct from the com- 

 pressed posterior umbonal slope, nor defined by any marginal 



* The type of Pernopecten, is P. cooperensis=(Avicula eooperensis, Shumard, =4vt'c- 

 ulopecten limaformis, White and Whitfield). The typical specimen of this species fig- 

 ured by Dr. Shumard in the Missouri Report, shows a few obscure, radiating costae — 

 an exceedingly rare character, though we have occasionally seen faint indications of 

 them on other specimens. Hence Dr. Shumard's species has not been generally iden- 

 tified, and Prof. Hall, in the Iowa Report, p. 522, pi. 1, fig. 9, erroneously refers appa- 

 rently the same shell to Avicula circulus, of Shumard, a quite different species. 



