CRUSTACEA. 
109 
Distribution. Upper Helderberg group. Corniferous limestone; Williams- 
ville, Erie county; Phelps and Canandaigua, Ontario county; Schoharie county. 
Proetus microgemma, n. sp. 
PLATE XXII, FIGS. 33, 34. 
Several detached pygidia afford certain characteristic features in vrhich they 
differ from described species. 
The shield is small and transversely semi-elliptical in outline, convex, 
longitudinally arched, distinctly and equally trilobate. 
The axis is elevated, tapering with slightly rounded margins to a blunt and 
somewhat elevated termination just within the border. It is somewhat ap- 
pressed at the sides just above the axial furrows, bears eleven annulations, 
which are angulated at the sides and posteriorly recurved over the median 
line. Upon the axial line each bears a strong tubercle. 
The pleura, are depressed below the axis, slightly flattened above and 
equally deflected to the lateral and posterior margins. Each bears seven or 
eight annulations which are grooved by fine impressed lines. 
The border is thickened, moderately and equally broad throughout its 
extent; it is encroached upon by the articulating ring, and slightly by the 
first two or three annulations. 
The surface is covered with minute and distinct tubercles, which are 
irregularly disposed upon the annulations of the axis, but are arranged in 
two or more rows on each annulation. A very young example, measuring 2 
mm. in length and 3 mm. in width, shows nine annulations on the axis and 
ten on the pleurae; the margin is less thickened than in the later stages of 
growth, all the pleural annulations encroaching upon it. The ornamentation 
is also more distinct than in larger specimens. 
An average specimen measures 6 mm. in length and 9 mm. in width. 
Distribution. Upper Helderberg group. Corniferous limestone: LeRoy, 
Genesee county; Canandaigua, Ontario county; Williamsville, Erie county; 
Falls of the Ohio, Ky. 
