REPORT OF THE STATE PALEONTOLOGIST 1902 905 



on the umbo. In one or two cases, traces of four muscular 

 impressions are seen on the dorsal cast. The specimens are very 

 perfect and probably more abundant than appears, as they 

 are so easily overlooked on account of their small size. They 

 are exact diminutives of the N. c o n c i n n a, resembling it in 

 every respect except size, adults in the pyrite being of about 

 one fifteenth the normal diameter. 



Localities. At the Livonia salt shaft, at Greigsville and at 

 Moscow near the Delaware, Lackawanna and Western Railroad. 

 Total specimens 10. 



Genus ambocoelia Hall 

 Ambocoelia umbonata Conrad, mut. pygmaea nov. 



Plate 2, fig. 13-15 



This is nearly circular in outline, the hinge line being very 

 short, not over half the width of the shell; the cardinal extremi- 

 ties are broadly rounded. An average specimen measures iy 2 

 mm in length, 1% mm in breadth and % to 1 mm in thickness. 

 The gibbosity varies greatly. The ventral valve is strongly 

 convex, with an elevated umbo, and a wide sinus from the 

 beak to the margin. This last is bounded by rounded ridges. 

 The small beak is incurved; the area is high, with a large 

 pedicle opening. The dorsal valve is nearly flat, the beak only 

 slightly raised, and just a trace of a low, wide ridge near the 

 margin corresponding to the sinus on the ventral valve. The 

 mutation resembles A. umbonata in most points, but differs 

 in that the sinus of the ventral valve is shallower and broader 

 in the presence of the fold on the dorsal valve, and in being 

 one tenth its diameter. In diameter the specimens are remark- 

 ably uniform, but in thickness there is considerable variation. 



Localities. This form is extremely abundant at Canandaigua 

 lake and the Livonia salt shaft, and appears scatteringly in 

 the Moscow localities. Total specimens 93. 



Ambocoelia umbonata Conrad, mut. pluto nov. 



Plate 2, fig. 16-18 



A second Ambocoelia, somewhat larger than the foregoing, 

 represents a phase corresponding to the A umbonata var. 

 g r e g a r i a , but is of earlier occurrence and is dwarf. This 

 is £y 2 mm in width, 3% mm in hight and 1% in thickness. It 



