72 NEW YORK STATE MUSEUM 



ventral margin, and a narrow inward beveled margin is observ- 

 able in other specimens. 



Dimensions. Length .9 mm, hight .6 mm, thickness .1 mm; 

 other specimens .11 x .7 x .1 mm; .11 x .8 x .1 mm. 



Horizon and locality. Extremely common in the more fine- 

 grained parts of the gray crystalline and of the compact reddish 

 gray limestone pebbles, where it, in spite of its very minute size, 

 belongs to the striking fossils by the pretty gloss on its deep 

 black valves. (Groups 6, 7) 



isochilina Jones 



Isochilina armata AYalcott var. pygmaea var. nov. 



Leperditia (Isochilina) armata Walcott. N. Y. 

 state mus. 35th an. rep't 1884. p. 213, pi. 17, fig. 10 



PI. 7, fig. 19-25 



About 20 specimens of this crustacean have been found in the 

 pebbles of compact reddish gray and black limestone and agree 

 in all essential features with that described by Walcott from the 

 Lowville and Black river limestone of Russia, Herkimer co. N. Y. 

 The greatest difference between this species and the variety from 

 Rysedorph hill is found in the relative size, Walcott's type 

 measuring 8.5 mm, while the specimens from Rysedorph hill only 

 range between .85 mm and 2,15 mm, the average' size of the 

 majority of the apparently mature specimens being 2 mm. 



Both forms have in common the elongate carapace, straight 

 hinge line, mucronate anterior and posterior cardinal angles. 

 The large type is farther described as having the anterior 

 extremity broadly rounded; ventral curve uniform, posterior 

 extremity obliquely rounded to the ventral curve, a relation 

 between the margins which also appears' in some of the speci- 

 mens figured, while in the great majority, as also in Walcott's 

 specimens, both anterior and posterior margins are nearly equally 

 rounded. 



The most characteristic feature of this species, and the one 

 which gave it its name, is a strong unciform spine, described aa 



