92 TVINCHELL AND MAKCY ON FOSSILS FROM THE 



scars a pair of small, feeble, diverging pyriform impressions, separated by a faintly raised 

 rostral septum, which reaches two fifths the length of the scars. Inside of valve showing 

 numerous faint, irregularly distributed radiating ridges which become visible about mid- 

 way between the beak and the margin. Sometimes one or two feeble concentric wrinkles 

 a little beyond the middle of the valve. The exterior is very differently marked. It pre- 

 sents a series of shallow, flattish-concave, radiating furrows, separated by abruptly raised, 

 narrow, sharply carinated costa?. The latter double in number near the middle of the 

 valve by implantation in the middle of the intervening furrows. 



Length of hinge-line of large specimen, 1.8 inch ; length of shell from beak to front 

 margin, 1.21 inch; transverse diameter of fissure, .39 inch; greatest width of area, .15 

 inch ; depth of ventral valve, .04 inch ; length of divaricator scars, .5 inch. 



In outline and surface characters of the casts of young individuals, this species resem- 

 bles Leptcena sericca Sow., but the divai'icator scars are very much smaller, differently 

 shaped, and more feebly impressed. It is also a leaner shell, and on the inside shows but 

 faintly the costal impressions which can be so plainly traced around the margin of that 

 species. It differs similarly from Streptorhynchus subplanus, its nearest analogue in rocks of 

 the same age. The characters of the exterior, and of the adult shell present, on the con- 

 trary, few resemblances to the species with which we have compared the young. 



Strophomena niagarensis W. and M. 

 Plate II. figure 9. 



Shell of large or medium size, somewhat hemispherical, hinge-line equal to the greatest 

 width, producing a semicircular outline. Ventral valve extremely ventricose, regularly 

 arched from beak to anterior margin, most elevated in the middle, flattened toward the 

 hinge extremities ; beak depressed, incurved, not surpassing the hinge-line. Area moder- 

 ately elevated in the middle, with a broad triangular foramen, situated nearly in the plane 

 of the commissure of the two valves, delicately striated transversely. Divaricator scars elon- 

 gate-ovate, but slightly divergent, reaching one third the length of the valve ; occlusor 

 scars narrowly linear, closely approximate ; rostral septum low, one third the length of the 

 divaricator scars. Internal surface of shell finely papillose in the region between the mus- 

 cular scars and the hinge-line ; the entire inner surface marked with very fine, irregular, 

 wavy stria?. Ventral valve sometimes with a shallow undefined sinus each side of a low 

 median ridge. 



Length of hinge-line, 1.58 inch; length of shell, 1.34 inch; convexity of the ventral 

 valve, .50 inch; length of divaricator scars, .65 inch. 



We have endeavored to unite this species with some of those already recognized. It 

 belongs to the group embracing &'. incequistriata, hemisphcerica, etc. which are the nearest re- 

 lated American forms. In outline it resembles the first, but the beak is less projecting, and 

 the stria3 are much finer. Unlike S. hemisphcerica it has no concentric wrinkles or other 

 markings ; and the hinge-line is proportionally longer, the muscular scars more elongate 

 and narrower, and the whole structure about the hinge is bolder and stronger. Neither does 

 8. hemisphceriea exhibit the internal granulations so conspicuous in our species. We think, 

 too, that our species is always even more ventricose than the other. Amongst known fos- 

 sils of the same age, there are none closely related. The resemblance of & svbplmms and 

 S. semifamda is quite superficial, and no more can be said of >S y . polenta of the Clinton 

 group. Amongst foreign species it approaches nearest to S. imbrex Pander and Davidson. 



