526 Systematic Paleontology 



moderately prominent, feebly prosogyrate, slightly anterior in position; 

 ligament area very small, and almost entirely in front of the umbones; 

 hinge and ventral margins parallel ; anterior lateral margin angulated dor- 

 sally, broadly rounded at the base ; posterior lateral margin obliquely trun- 

 cate; external sculpture of very fine, flattened, radial threadlets, forty to 

 fifty in number, least fine and most flattened laterally particularly upon 

 the posterior slope; interradials linear; concentric sculpture manifest in 

 minute corrugations of the radials particularly upon the disk; hinge teeth 

 laminar, parallel or subparallel to the hinge line; anterior teeth parallel 

 to the hinge, rather short, that nearest the hinge margin a little longer 

 than the two beneath it ; posterior teeth also three in number, discrepant 

 in size and slightly oblique to the hinge ; medial lamina the longest, pro- 

 duced beyond the distal extremity of the cardinal line ; the one dorsal to 

 it merely the locally elevated margin of the hinge plate, that ventral to it 

 also produced beyond the cardinal margin but not more than half the 

 length of the medial lamina, disappearing within the umbonal cavity; 

 characters of cicatrices and pallial line obscure. 



Gabb's type is in the collection of the Philadelphia Academy of Natural 

 Sciences. 



Johnson (see synonymy) was the first to call attention to the fact that 

 the form .described and figured by Conrad ' was not the Area (Macrodon) 

 eufalensis of Gabb, but a distinct species to which he suggested that the 

 name Nemodon conradi be assigned. 



This Nemodon conradi is separated from the true X. eufalensis by the 

 less elongated outline, the more nearly central umbones and by the pres- 

 ence of a fine radial sculpture over the entire external surface, the radials 

 least elevated on the medial portion. 



Occurrence.— Monmouth Formation. Brightseat, Brooks estate near 

 Seat Pleasant, and McNeys Corners, Prince George's County. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, U. S. National Museum. 



1 Conrad, 1869, Amer. Jour. Conch., vol. v, p. 97, pi. ix, fig. 16. 



