302 Canadian Itecofd of Science. 



just outside of the northern boundary of Manitoba, J. B. 

 Tyrrell, 1890 ; a single and not very well preserved cast of 

 the interior of the dorsal or upper side, in a compact and 

 slightly vesicular dolomite. Although the surface markings 

 are not even faintly indicated, and the characters of the 

 glabella and some of those of the central portion of the tho- 

 rax are unknown, the whole of the marginal outline of the 

 specimen can be ascertained with considerable accuracy. 



In the elucidation of its characters the writer has been 

 materially assisted by Mr. L. M. Lambe. 



The species appears to be of the type of the A. Prevostii 

 ofBarrande, 1 from the Upper Silurian Bocks (Etage E.) of 

 Bohemia, but it has a smaller number of short spines on the 

 lateral margins of the two free cheeks, a proportionately 

 broader axis to the thorax, much longer primary spines on 

 the pygidium, and differs from that species in several other 

 particulars. 



EXPLANATION OF PLATE III. 



Strophomena acanthoptera. 



Fig. 1. — Dorsal view of a specimen, showing the whole of the dorsal 



valve and the cardinal areas of both valves. 

 Fig. 2. — Yentral view of another specimen. 



Pbntamerus decussatus. 



Fig. 3. — Dorsal view of a specimen, showing the whole of the dorsal 

 valve and the prominent umbo and recurved beak of the 

 ventral. 



Fig. 4.— Outline of the posterior end of a cast of the interior of the 

 shell of this species, to show the relative convexity of 

 the two valves, the length and other characters of the 

 mesial septum of the ventral valve and the nature of the 

 two short septa in the dorsal. 



GoMPHOCKRAS PARVULUM. 



Fig. 5. — Lateral outline of a cast of the interior of the shell. 



1 Systeme Silurien du Centre de la Boheme, Prague and Paris, vol. i, 1852, p. 

 739, pi. 39, figs. 33-41. 



