JUKASSIC FOSSILS. - 355 



unci flic beak small, but often approaching tumid, the apex incurved and 

 extending- a little above tlie hinge-line ; anterior wing small, al)ruptly 

 rounded toward the beak, its surface strongly curved toward the opposite 

 valve near the cardinal margin, so as to leave a small byssal opening, or gap- 

 ing at the anterior margin; posterior wing longer and somewhat flattened 

 toward the extremity; surface of the valve marked by raised radiating lines, 

 Avhich are very variable in number and character on different individuals, 

 and also by concentric lines which often form slight asperities or nodes in 

 crossing the radii, especially on the sides and near the basal border of well- 

 preserved specimens; right valve slightly less convex than the left, with a 

 smaller and less conspicuous beak, which does not extend beyond the cardi- 

 nal line; anterior side characterized by a short and very narrow wing, sep- 

 arated from the body of the shell by a sharp, deep, and very narrow groove, 

 the actual sinus scarcely extending within the limits of the wing, while the 

 anterior margin of the valve below extends considerably beyond the ex- 

 tremity of the wing in a nearly parallel direction ; posterior wing propor- 

 tionally larger and compressed, the extremity angular and sometimes almost 

 pointed, the posterior border of the valve running nearly at right angles to 

 the hinge line,. or slightly sinuate below; surface of the valve marked by 

 strong concentric lines of growth, and on the body of the shell by obscure 

 radii, which are often obsolete. Each valve is further characterized by a 

 narrow, longitudinally striated ligamental area, extending nearly or quite 

 to the extremities of the hinge line; and in the right valve the area is nar- 

 rowed anteriorly, for the accommodation of the internal, slightly oblique 

 fold of the shell, forming the depressed groove between the ear and the 

 body of the shell. On the left valve there is a slight depression or break 

 in the ligamental area beneath the beak, and an apparent depression on the 

 right valve (these features of the area are obscure and a little uncertain on 

 account of the small size of the specimen). Muscular impression as seen 

 on a right valve rather large, and subcentrally situated. 



We have considerable doubt as to any very close relations of this spe- 

 cies to the genus Pseudomonotis, as typified by Monotis speluncaria Munster, 

 or P. liaivni M. & H. All the Upper Carboniferous specimens of the latter 

 species which we have examined appear to have a distinct notch or sinus 



