50 GEOLOGICAL AND NATURAL HISTORY SURVEY OP CANADA. 



Aparchites unicornis, Ulrich. (Var.) 

 PL IX., fij?. 11. 



Leperditia unicornis, JJlrich. 1879. Journ. Cin. Soc. Nat. Hist,, vol 2, p. 10, 

 pi. 7, figs. 4-46. 



A single valve which agrees too closely with A. unicornis to be dis- 

 tinguished therefrom specifically, adheres to one of the Stony Moun- 

 tain slabs. It differs from the original examples of the species, which 

 €ame from the lower or Utica horizon of the Hudson River or Cin- 

 cinnati group, in having the two ends less equal and the spine a little 

 stronger. 



This species is not a Leperditia, as its valves meet without overlap- 

 ping. There seems to be no reason for doubting that it is congeneric 

 with the group of "nonsulcate Primitia," and should, therefore, be 

 placed under the new genus Ararchites, Jones. 



Primitia lativia. (N". Sp.) 

 PI. IX., fig. 



Valves rather strongly convex, suboval, slightly oblique, the height 

 and length respectively, as nine is to thirteen. Ends nearly equally 

 convex. Dorsal margin straight, its length equalling two-thirds that 

 of the valve. Ventral margin convex, merging more gently into the 

 posterior than into the anterior margin. Sulcus wide, strongly im- 

 pressed, extending from the dorsal margin halfway across the valve. 

 Posterior border of sulcus more abrupt and more elevated than the 

 anterior. Point of greatest convexity just below and a little behind the 

 centre of the valve. End view of carapace ovate in outline. No distinct 

 border is present, but the margin frequently flares a little. 



Length of an average specimen, 1.35 mm. ; height, 0.91 mm. ; great- 

 est convexity of the valve, 0.3 mm. 



This is a true Primitia, with rather close relations to P. bivertex, 

 Ulrich. The sulcus, howevei*, is unusually wide, and that species has 

 two strong nodes near the dorsal mai-gin situated one on each side of 

 the sulcus. Of European species P • renulina, F. fabulina, 3ind certain 

 varieties of P. mundula, all of Jones and Holl, should be compared 

 with it. None of these species, however, have so wide a sulcus. 



The species is represented by several valves attached to slabs of 

 limestone from Stony Mountain, collected by Dr. Ells, in 18^5 and by 

 T. C. Weston in 1884, it also occurs in the upper beds of the Hudson 

 River or Cincinnati group at several localities in southeastern Indiana. 

 At a locality several miles north of Madison, Ind., several specimens 

 were found associated with large numbers of Leperditia ca'cigena, 

 Miller. 



