162 CONTRIBUTIONS TO CANADIAN PAL^ONTOLOGT. 



The Aptijchus or portion of an Aptychus represented on Plate 21, (fig. 

 G) was found in the same beds as the S. boreatis, and may have belonged 

 to that species. It is longitudinally semiovate in outline, and its outer 

 surface is marked only with rather closely disposed concentric raised 



CRUSTACEA. 

 PHYLLOPODA. 



BSTHERIA BELLULA. (N. Sp.) 



(Plate 21, figs. 7 and 7a.) 



Carapace valves compressed at the sides, but regularly though 

 moderately convex (so that a transverse section of both when closed 

 would be ovately ^nd narrowly lenticular in outline), inequilateral, a 

 little longer than high and varying in outline in different specimens 

 from obliquely subovate to longitudinally semiovate. Anterior end 

 always shorter and generally narrower than the posterior. 



The most perfect specimen collected, which is figured on Plate 21, 

 fig. 7, and which may be i-egarded as the type of the species, is ob- 

 liquely subovate in marginal outline. Its anterior and posterior ends 

 are both rounded, as ai'e also both of its cardinal angles, and its dorsal 

 margin is comparatively shoi't, though more than half the entire 

 length. In one of the longitudinally semiovate examjjles (fig. 7 a) 

 the anterior end is angular at its junction with the dorsal margin 

 above, and much narrower than the posterior end, which latter is 

 rounded both above and below; while in another, the marginal out- 

 line is not far from semicircular, the dorsal border or hinge line 

 extends nearly the whole length of the valves, and is angular at both 

 ends. Umbones small, depi'essed, contiguous and placed near the an- 

 terior end, but not quite terminal. 



Surface marked by from thirteen to eighteen closely and rather 

 regularly disposed concentric ridges, which are rounded and not very 

 prominent, althougli distinctly defined. 



Dimensions of one of the most perfect specimens collected : maximum 

 length, seven millimetres : greatest height, five. 



One perfect and well preserved cast of the interior of a pair of valves 

 which had become widely spread out, a few similar but not quite so 

 well preserved casts of detached valves, and a single cast of the exterior 

 of a left valve. 



