184 PROCEEDINGS OP THE ACADEMY OF 



at first perpendicularly, after which they are deflected forward parallel to the 

 basal and anterior borders. Length and height, each about 097 inch ; con- 

 vexity 0-58 inch. 



Locality and position. Sonth-west base Black Hills, Jurassic. 



Named in honor of Mr. T. A. Conrad, the well known palaeontologist, of 

 Philadelphia. 



Pecten extenuatds, M. & H. Shell broad ovate, or sub-circular, thin, 

 compressed; basal mtrgin rounded; beaks small; hinge line rather short; 

 ear3 unknown ; surface apparently having only concentric striae of growth. 

 Height 0-98 inch, length 0-90 inch; convexity 0-28 inch. 



Locality and position. South-west base of Black Hills, in a sandstone of lower 

 Jurassic age. 



PALEOZOIC. 



Myalina aviculoides, M. & H. Shell subtrigonal, higher than long, very 

 convex, or sometimes subangular down the umbonal slopes ; anterior margin 

 distinctly sinuous above the middle, thence descending with a slightly convex 

 curve, nearly at right angles with the hinge, to the basal extremity, which is 

 narrowly rounded ; posterior side compressed, its margin curving a little 

 forward above, or intersecting the hinge at right angles, slightly convex, and 

 nearly perpendicular along the middle, below which it curves obliquely forward 

 to the abruptly rounded basal extremity; hinge straight, nearly equalling the 

 length of the shell ; beaks very convex, subangular, and curving rather abruptly 

 forward, so as to become nearly, or quite terminal ; surface having moderately 

 distinct marks of growth. Length, 1-48 inch ; height, 1-66 inch ; convexity, 

 (of left valve), 0-32 inch. 



This will be readily distinguished from all the other species of the genus 

 known to us, by its more accurate front, and the extension of its anterior margin 

 under the beaks, above its most sinuous part. 



Locality end position. From the upper beds, containing Permian types of 

 fossils, on Cottonwood creek, Kanzas Territory. 



Note. In going carefully over these extensive collections, we have in addition 

 to finding the new species here described, succeeded in working from the matrix, 

 better specimens of many of those already published by us, than had been pre- 

 viously obtained. The additional information derived from these, and a more 

 careful review of the subject has enabled us to make several corrections 

 in the synonyma, as well as in the generic references, a list of which is given 

 below. 



It will also be observed, that we have made quite a number of other changes, 

 in order to range the species under the oldest generic names proposed after the 

 introduction by Linnaeus, of the binomial system. We must confess, however, 

 that we have some doubts whether science is to be much benefitted by a strict 

 observance of the law of priority, in such cases as those where it becomes 

 necessary to change long established names. We nevertheless make some 

 such changes in conformity with usages rapidly gaining ground, and probably 

 destined soon to become universal amongst conchologists and laborers in other 

 departments of Natural History. 



The transfer of several species formerly published under the names Hamites, 

 Ancylocerasl and Turrilites, to the genus Helicoceras, has been made in accord- 

 ance with the views of Mr. Daniel Sharpe, (Fossil Mol. Chalk, England, part 

 3d, Cephalopoda, p. 59, Paleont. Soc.) who refers all the so called Turrilites 

 having rounded whorls, with the siphuncle placed on the dorsal or outer side, to 

 the genus Helicoceras, whether the whorls are in contact or not. The genus 

 Turrilites, he restricts to those forms having more or less angular contiguous 

 whorls, with the siphuncle located near the suture. The fact of the whorls of 

 those forms with rounded volutions being in contact or not, can scarcely be 



[May, 



