192 NEW YORK STATE MUSEUM 



front, with straight or gently concave sides, and extends about half the length of the shell. 

 The width of this space, in a specimen thirty lines wide on the hinge line, is thirteen 

 lines across the anterior angles. Its length the same. In a smaller specimen, sixteen 

 lines wide on the hinge line, the length of the space is about seven lines, its width at 

 the front five lines. The divaricators are divided into a number of longitudinal lobes, 

 seven or eight in large individuals, fewer in the smaller, and sometimes very indistinct. 

 The occlusors are narrow and half the length of the other two, very indistinct. The 

 mesial septum sometimes extends the whole length of the muscular impressions, and 

 becomes stronger on approaching the beak; in the small specimens it is very slight or 

 absent altogether. 



Surface with fine, rounded radiating striae, ten or twelve in the width of two lines. 

 These are crossed by very fine concentric striae, which disappear when the shell is slightly 

 worn or exfoliated. In the cast of the inner surface of the ventral valve, all the space 

 between the muscular impression and the cardinal angles is punctured. All around the 

 sides and front margin the punctures are very small, and partially arranged in rows con- 

 forming to the radiating striae. 



The specimens as yet collected are not in a condition to show the area perfectly. It 

 is, however, vertically striated and denticulated throughout nearly, if not entirely, the 

 whole length. There seems to, be a small triangular deltidium with a cavity beneath it. 



Width of the largest specimen collected forty-two lines; length of the same three 

 inches; dorsal valve unknown. 



In S. blainvillei the divaricators extend nearly two thirds the length of the 

 shell; in this species about half the length. It approaches S. magnifica in size, but 

 the form and proportional length of the muscular impressions are different in the two 

 species. It is closely related to S. beckii Hall, but attains a much larger size; is not 

 concentrically wrinkled; has finer radiating striae and a deltidium. S. irene has only 

 four or five radii in the width of two lines while this species has ten or twelve. 



Locality and formation. Mount Joli and Split Rock, Perce. Lower Devonian. 



This is the prevalent Leptostrophia at Perce and may be regarded as 

 the local variant of L. magnifica as the points of difference though at 

 once recognizable are relatively few. These are chiefly the absence of 

 oblique crenulations near the hinge angles and a difference in the character 

 of the striae ; we need put little emphasis on variations in muscle scars for 

 these in large examples are hardly to be distinguished from those of L. 

 magnifica. Billings, however, apparently included two distinct forms 

 under his term S. t u 1 1 i a and in the rarer of these which we describe below 

 there is a prevailing uniformity of surface striation which is in contrast to 

 that of the species under discussion. The latter has not the regular thread 

 lines as described by Billings, but highly irregular lines almost alternating 

 in size or tending to fasciculation with a group of lesser ones between a pair 

 of stronger. These differences are brought out in our illustrations. The 

 prevailing size is rather small ; large shells are infrequent. 



Locality. Very abundant at Perce Rock, frequently covering entire 

 slabs. 



