Monograph of the LameUihranchiata. 229 



In some species the teeth on either side of the beak curve outward 

 from it, and in others inward, toward the beak on both sides. The 

 teeth are often very minute immediately beneath tlie beaks. 



The shells of this genus vary from elliptical to ovate and subtrian- 

 gular forms, many of them being contracted on the posterior side ; 

 they are usually of moderate thickness, though one S2:)ecies is very 

 thick and strong. Some of the species have a distinctly impressed 

 lunule. The lesser muscular impression • is often a small pit, placed 

 directly beneath the hinge line, and between it and the large nmscular 

 impression. The beaks are usually of medium size, pointed, rarely 

 ventricose, approximate or in contact, never sub-spiral. 



The relations of this shell are among the Avcadce and approximate 

 to the Nucala in their general character, and to which genus they have 

 usually been referred. They differ from that genus, however, in the 

 absence of the ligaraentary pit beneath the beak, and in the presence 

 of an external ligament and double muscular impressions. 



Tellinomya (?) ohliqua — (Hall, 1845) as redefined by jNIeek, 



Shell very small, compressed, subcircular, appi'oaching subquadran- 

 gular ; height and breadth about equal ; anterior margin short and 

 rounding into the rounded basal margin ; posterior margin subtruncated, 

 or more or less rounded; beaks elevated, nearer the anterior margin ; 

 dorsal margin sloping from the beaks, the anterior slope being the 

 more abrupt, and the margin behind the beaks straighter, more 

 compressed and sharper ; sui"iace smooth ; internal casts showing the 

 muscular impressions to be, comparatively, rather distinct. Hinge 

 unknown. 



Length of a medium sized specimen, 0.06 inch ; height slightly 

 less ; convexity, 0.03 inch. It varies, however, from less than half 

 this size to more than double as large. 



It is found throughout the Cincinnati Group, from low water-mark in 

 the Ohio river t(j the Upper Silurian rocki^. It should be regarded as 

 a common fossil. 



, Tellinomya pedunculoides — (Hall). 



Shell small, sub-circular in outline, with the posterior end slightly 

 prolonged below the middle, giving a little obliquity to the shell ; pos- 

 terior cardinal border obliquely sloping to the point of greatest exten- 

 sion ; anterior and basal borders regularly rounded ; beaks small ; 



