FOSSILS OF THE HUDSON RIVER GROUP. 87 



die ; basal line gently curved, becoming a little sinuate at or about the 

 anterior third of its length ; anterior end narrowly rounded. Beaks 

 small, and compressed on the back, projecting but little above the hinge. 

 Surface of the valves moderately convex when not distorted by pressure ; 

 most prominent about the umbonal ridge, which is low, and broadly 

 rounded ; not forming a conspicuous feature of the valve. A very slight 

 and rather undefined mesial sulcus crosses the valves from the beak to 

 the sinus of the basal margin. 



Surface of the shell marked on the cardinal slope and posterior end by 

 regular, even, concentric furrows, from three to four of which occupy the 

 space of an eighth of inch in their strongest parts. These furrows are 

 most distinctly marked near the cardinal margin, and become obsolete 

 in crossing the umbonal ridge ; existing on the basal portions and ante- 

 rior end only as fine, irregular, concentric striae of growth. Anterior 

 muscular impression strongly defined and proportionally large ; forming 

 a rather distinct, sub-circular or reniform protuberance on the anterior 

 end of the casts ; posterior impression not observed ; pallial line often 

 distinct on the anterior half; partly composed of detached transverse 

 pustules. 



The specimens of this shell observed in collections have generally 

 been found among and considered as identical with those of Modiolopsis 

 modiolaris, but it is always of much smaller size, being generally not 

 more than two-thirds as long when fully grown as the ordinary sized in- 

 dividuals of that species, and although the general form is very similar, 

 the concentric markings of the cardinal slope and posterior end readily 

 distinguish them. These markings are of such a nature that they are 

 usually preserved on all specimens retaining any specific markings. 

 The specimens are frequently much distorted by compression, and when 

 the pressure has been vertical, or in the direction of the plane of the 

 shell, the convexity is considerably increased, often causing them to ap- 

 pear nearly cylindrical. 



The species resembles very closely Modiomorpha concentrica of the Ham- 

 ilton formations of New York (Modiola concentrica of authors) in its gen- 

 eral appearance and surface markings, so much so, that it might readily 

 be mistaken for that shell ; but the concentric, undulating striae becom- 

 ing obsolete on the umbonal and anterior portions of the shell, will serve, 

 We think, as an unfailing means of distinguishing them — the striae on 

 that species continuing over all parts. 



Formation and locality: In the softer layers of the Hudson River group, near 

 Waynesville, Ohio. Received from Jesse Van Duser, Esq., of that place. We have 

 seen specimens labeled Cincinnati, Ohio, and perhaps it may also occur at that locality. 



