EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 237 



character of fimbriae than of the Hnes on S p i r i f e r r a d i a t u s Sow. 

 which, without phcations, is the starting point of a considerable series of 

 radiate-phcate shells. The general approach of both our specimens to 

 species of Athyris, such as A. s p i r i f e r o i d e s of the Hamilton fauna 

 indicates a more probable relation therewith. 



Leptostrophia blainvillii Billings (sp.) 

 (=L e p t o s t r o p h i a p e r p 1 a n a Conrad ?) 



Plate 36, figures 19-24 



Strophomena blainvillei Billings. Palaeozoic Fossils. 1874. v. 2, pt i, 

 p. 28, pi. 2, fig. I, a-b; pi. 3, fig. I 



Original description. Shell semielliptical, hinge line equal to a little less or a little 

 greater than the width; sides in the posterior half usually gently concave, sometimes 

 straight or gently convex; front broadly rounded, sometimes straightish in the middle; width 

 from one eighth to one sixth greater than the length. 



Ventral valve gently convex; greatest elevation about one third the length from the 

 beak; cardinal angles compressed; the umbo broadly obtuse; beak scarcely distinct from 

 the cardinal edge; area flat, inclined at an angle of about 40° to the plane of the lateral 

 margin, striated and with the edge denticulated its whole length; dorsal valve nearly flat, 

 slightly concave; area very narrow; both valves usually with several concentric wrinkles 

 or rugae of growth. 



.Surface covered with fine, rounded or subangular radiating striae, of a very uniform 

 size; about twelve in the Avidth of two lines. When slightly exfoliated the grooves between 

 the elevated striae are seen to be regularly pitted or punctated, from six to eight punctures 

 in one line. 



In the interior of the ventral valve the area occupied by the muscular impressions is 

 of a subtriangular shape, and extends from the beak about two thirds of the length. The 

 front of this area is generally broadly rounded; the two upper sides converging with a con- 

 cave curve to the beak. The space on each side of the upper part of the area is strongly 

 tuberculated, becoming gradually smooth towards the cardinal angles. A thin mesial sep- 

 tum extends from the beak two thirds the length of the shell. For the length of about a 

 line from the beak the septum is much thickened, leaving in the cast of the interior a 

 short, deep fissure, as shown in figure i. On each side of this fissure there are two tuber- 

 cles, more or less elevated, which are the casts of the cavities in the shell, for the reception 

 of the divaricator processes. The divaricator scars are divided into a number of lobes, 

 usually five or six, but often more. The occlusors are rather small, and extend from the 

 thickened part of the septum about one third the length of the divaricators. All around 

 the sides and front of the muscular area the casts of the interior are striated, the striae 

 often becoming coarser as they recede from the margin. 



The casts of the interior of the dorsal valve exhibit two ovate pits close to the beak 

 [fig. lb], the impressions of the divaricator processes. From these two pits the impression 

 of a short septum extends, ending in a point at about three lines from the beak. The 

 occlusor scars are situated on each side of this septum, but their form is not exhibited in 

 any of the specimens collected. The shell on each side of the scars is coarsely tubercu- 

 lated, smooth along the hinge line and at the cardinal angles The divaricator processes, 

 as shown by their impressions, are short, erect, slightly sloping backwards, but diverging 

 laterally. 



None of the specimens are sufficiently perfect to exhibit all the details of the area. 

 In some the casts show that there is a small triangular pit beneath the beak, as there is 

 in S. m a g n i f i c a. 



