EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA I93 



Leptostrophia Irene (Billings) 



Plate 38, figure 3; plate 39, figures 5-8 



Strop ho men a irene Billings. Palaeozoic Fossils. 1874. v. 2, pt i, p. 27, 

 pi. 2, fig. 5, 5a 



Original description. Shell large, nearly flat; cardinal angles usually slightly project- 

 ing; sides in the posterior half either straight and subparallel or gently concave and con- 

 verging towards the front; anterior half broadly rounded; front margin sometimes nearly 

 straight in the middle. Width on the hinge line usually from two to three inches; length 

 from one sixth to one third less than the width. 



The ventral valve is very slightly and uniformly convex, most elevated about tlie 

 middle or a little above, compressed towards the cardinal edges; umbo and beak small; 

 area about two lines high at the beak in a s])eciraen 2^ inches wide inclining backwards 

 at an angle of about 45" to the plane of the lateral margin. Dorsal valve gently concave, 

 conforming in its curvature to that of the ventral valve; area half a line wide^ forming 

 nearly a right angle with the ventral area. The muscular impressions of the ventral valve 

 are rather large, flabellate, extending from the beak nearly half the length of the shell. 

 The divaricators (in the only specimens in which they have been seen) are divided into six 

 or seven longitudinal lobes, not distinctly defined at their anterior margins. The occlusors 

 occupy an elongate, narrow space, and seem to extend from near the beak to near the f ron t 

 of the divaricators. There is a slightly elevated mesial septum between the occlusors, one 

 half or two thirds their length from the beak. Hinge line crenulated, apparently nearly 

 out to the end. Deltidium and internal characters of the dorsal valve unknown. 



Surface with coarse, flexuous, slightly elevated, rounded, radiating striae, which 

 increase, both by bifurcation and intercalation, four or five in the width of two lines. When 

 the surface of the shell is perfect, it exhibits a set of fine but very distinct concentric 

 striae, six or seven in the width of one line. When partially exfoliated these become 

 obscure, or disappear altogether. The shell is not punctate. 



The differences in L. irene and L. magnifica are readily appre- 

 hended. The coarse striae of the former are specially noteworthy and for 

 comparison we note their number as follows : in a width of 5 mm at 10 mm 

 from the beak, 8 ; at 20 mm, 5 ; 30 mm, 5 ; 40 mm, 5 ; 50 mm, 6. This 

 statement may be compared with that given for L. magnifica. Further, 

 the interstrial grooves are broad, the concentric lines very obscure, the 

 oblique cardinal crenulations generally absent, faint if present. Of the inte- 

 rior characters Billings has shown that the muscle scar of the ventral valve 

 is shorter or less expanded than in L. magnifica and we observe that 

 the pallial region is much more sparsely pustulose and the cardinal denticu- 

 lations are smaller. The dorsal valve was not described by the author cited. 

 We find it in a small specimen to be almost if not quite flat but the interior 

 has not been observed. 



Di7nensions. Length of largest specimen, 64 mm; width on hinge, 75 

 mm. Other specimens measure 58 and 46 mm in length, 68 and 62 mm 

 along the hinge. 



Localities. This species is less common in the limestone fauna than 

 L. magnifica and has not yet been found in association with it. Our 



