LOWER SILURIC SHALES OF THE MOHAWK VALLEY IO3 



Remarks. This small type appears to us to be most similar to 



C . m e d i a 1 i s Ulrich among the western Trenton forms, 



with which it has the general outline and the position of the beaks 



! in common. Our species is, however, more elongate and possesses 



a more convex anterior margin. 



Ctenodonta subcuneata nov. 

 PL 6, fig. 9, 10 



Description. Shell below medium size, compressed, only about 

 one-eighth longer than high, and about one-eighth as thick as long, 

 acuminate subovate in outline; umbones fairly prominent and beaks 

 incurved, directed distinctly forward and situated between one-third 

 and one-fourth of the length behind the anterior extremity ; posterior 

 and anterior ends about equally wide, anterior dorsal margin oblique, 

 slightly concave, posterior dorsal slope slightly convex, postero-basal 

 part well rounded and basal margin broadly rounded. Umbonal 

 ridge broad and distinct only in umbonal region. Surface marked 

 by a few varices of growth and fine concentric lines. Posterior 

 muscle scar faintly seen. Hingle plate not seen. 



Position and localities. In dark gray sandy shale of Snake Hill 

 beds, Snake hill, Saratoga county, N. Y. 



Remarks. This species is easily distinguished from the associated 

 congeners by its rather high relatively flat shell. According to a 

 note from Doctor Ulrich it is allied to the western Richmond species 

 C. fecunda (Hall). It is relatively higher and less extended 

 posteriorly than the latter. 



Allodesma subellipticum Ulrich 

 PI. 6, fig. ii 



The Canajoharie shale above the falls at Canajoharie has afforded 

 several specimens of a small thin-shelled pelecypod that are much 

 flattened and fail to show any structure. Doctor Ulrich has referred 

 them doubtfully to his Allodesma subellipticum, a 

 species from the Galena shales of Minnesota. 



Lyrodesma schucherti nov. 

 PI. 6, fig. 12 



Description. Shell elongate-ovate, twice as long as high, greatest 

 height a little anterior to middle ; anterior part drawn out, posterior 

 one obliquely truncate; shell rather flat, greatest thickness subcen- 



