yetc iSjH'cu's of Fossi/x ffoni Oh hi. 1!)7 



ill being i)roi)ortioii;dly miicli sliorler, which gives it ;i more 

 semicircular form. The eye-tubercles are iilso more iieiirly of 

 the size of those of that species and similarly situated. 



The thorax closel}' resembles that of E. remipes in its genei'al 

 form, but the lower three of four segments are proportionally 

 shorter, giving the posterior extremity a much more coin])act 

 character. The principal distinction between the two species, 

 iis shown by the thorax, exists iu a difference of the ornamenta- 

 tion of the surface, as seen on the specimen used. This consists 

 in the minute spine-like pustules or pointed granules, marking 

 the surface of the crust, beino- arrano-ed in irreo'ular transverse 

 lines across the body, and parallel to the anterior and posterior 

 margins of the segments, instead of being irregularly disposed, 

 as in all other species described. No indication of the longitu- 

 dinal rows of larger pustules, marking the median line of the 

 thoracic segments, can be traced. Caudal spine not observed. 



Leperditia angulifera, n. sp. 



Pal. O., Vol. Ill, Plate 1, Figs. 28— ;J0. 



Carapace of medium size, having a length, iu adult individuals, of about 

 three-eighths of an incli, by a height of one-fourth of an inch in tlie broadest 

 part. General form of the outline broadly sub-ovate and widest posteriorly; 

 hinge-line straight, equal in length to two-thirds that of tlie entire valve ; 

 anterior end a little tlie shortest, narrowly rounding into tlie broadly curved 

 basal line ; posterior end broadlj" rounded. Surface of the carapace highly 

 elevated and prominent, forming a strong, somewliat angular, longitudinal 

 node just within the basal margin, and near the middle of the length. From 

 this point, the surface slopes somewhat gradually upward to the hinge-line, 

 witli a barely perceptible convexity, except on tlie anterior end, wliere it is 

 more strongly convex, and cliaracterized by a rather prominent and well- 

 marked ocular tubercle. From the angular nods near the lower margin, 

 there is, on well-preserved individuals, a perceptible angulation, extending 

 along the surface to the point of greatest length on the anterior end, and a 

 similar one, but less strongly marked, on the posterior side. There is no 

 perceptible difference in form between the right and left valves, each show- 

 ing the features about equally developed. No appearance of striations ra- 

 diating from the ocular tubercle can be detected, eitlier on the internal 

 casts or in the matrices ; still the nature of the rock in which they are im- 

 bedded is such tliat very obscure markings would scarcelj' be preserved. 



This species differs from Leperditia alta, Conrad, of the same 

 formation, in its larger size, and in the larger and more distinct 



