LOWER SILURIC SHALES OF THE MOHAWK VALLEY 97 



nating in about the middle of the umbonal ridge. Dorsal and 

 ventral margins distinctly diverging posteriorly, the former straight 

 or but very gently arcuate, the latter with a slight and broad sinus 

 in the middle. Anterior end narrow, produced, the posterior end 

 well rounded and quite convex above the somewhat more abruptly 

 rounded basal part and passing gradually into the hinge line. Beaks 

 not prominent, compressed, about one-fifth of the length of the shell 

 from the anterior extremity; umbonal ridge subcarinate, a very 

 marked feature above and traceable to the postero-basal margin. 

 Mesial sulcus well defined and an obscure broad ridge passing from 

 the beak obliquely forward and downward to the antebasal angle. 

 Surface with irregular undulations and fine intercalated lines. Mus- 

 cular scars not distinct. 



Position and locality. Rather common in a bed of dark gray 

 sandy shale of the Snake Hill beds at Snake hill, Saratoga county, 

 N. Y. 



Remarks. Doctor Ulrich writes me regarding this species that it 

 belongs to an early group of Modiolopsidae, combining, with some 

 of its own, the characters of Modiolopsis and Orthodesma, adding : 

 " Its nearest ally is O. subnasutum. Another close ally 

 is the Modiolopsis milleri (Cincinnati), a third 

 M. a r gut a Ulrich (Black River). Several other species of 

 the group which will some day be distinguished generically are 

 figured on plate 36 of my Minnesota report. The Snake Hill species 

 differs from all in its more sharply defined umbonal ridge. None 

 of the specimens retains its original outline. One (plate 5, fig- 

 ure 5) comes nearest the normal." We have used the latter as 

 type of the species and based our description upon this specimen. 

 The specimen (plate 5, figure 7) is clearly too elongate by 

 lateral compression; there occur specimens which possess not half 

 that relative length and are compressed in antero-posterior direction. 

 It also appears to us that O. subcarinatum possesses a 

 more evenly rounded posterior margin than O. subnasutum 

 and is relatively a little higher posteriorly, and is smaller; but 

 otherwise it is hardly distinguishable from the western Galena 

 form. 



Prolobella ? trentonensis (Conrad) 



PI. 5, %. 4 



This species was first described as Avicula trentonen- 

 sis by Conrad (Jour. Acad. Nat. Sci., 1842, 8:240) from the 

 Trenton limestone and shortly after (title 3, page 161) by Hall 



