CRUSTACEA. 109 



Distribution. Upper Helderberg group. Corniferous limestone: Williams- 

 ville, Erie county ; Phelps and Canandaigua, Ontario countj ; Schoharie county. 



Proetus MICROGEMMA, 11. sp. 



PLATE XXII, FIGS 33, 34. 



Several detached ptgidia afford certain characteristic features in which they 

 differ from described species. 



The shield is small and transverse^ 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, bear- 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 ami 

 equally deffected to the lateral and posterior margins. Each bears seven or 

 eight annulations which are grooved by line impressed lines. 



The bonier 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 angulation. 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. 



