JONES ] PALAEOZOIC OSTRACODA. 6t 



tion, from Ottawa * and Nepean f, Ontario, and Pointe-aux-Pina, 

 Aylmer ;[, Province of Quebec, and constituting in each case nearly all 

 the rock-mass, are very similar among themselves, and approximate 

 closely to Isochilina Ottawa. There are some slight differences, how- 

 ever, from that species. The outline is not so oblong, the postero- 

 dorsal angle being less prominent, and the antero-ventral margin rather 

 less fully curved. The eye-spot is more isolated and distinct in some 

 than in others, and the sulcus or depression behind it is variously 

 developed, but always stronger than in the original specimens. The 

 radiating vascular lines (seen only in the casts formerly) stream off 

 backwards from the muscle-spot ; otherwise the valves are smooth. 



In shape some of these specimens (see figs. 10 and 11) approach /. 

 gracilis, Jones, but the ventral margin is too boldly and deeply curved. 

 The surface is not punctate, and individuals with pitted margins are 

 rare. 



As they agree with neither I. Ottawa nor /. gracilis in all respects, 

 and come nearest to the former in shape, 1 regard them as representing 

 an intermediate variety and call it J. Ottawa var. intermedia. 



The species was first obtained from the Calciferous Sandiock at 

 Grenville, and the Chazy Limestone of L'Original, in both instances 

 forming an Isochilina-limestone, as at Ottawa, Nepean, and Aylmer. 

 At L'Original and Grenville the individuals vary much in size, some 

 having a length of six mm., others only three mm., and some are rela- 

 tively shorter and higher than others. As it is so variable a species, 

 we are not surprised at finding a noticeable variety with close affini- 

 ties to the known types. 



Isochilina gracilis, Jones (Ann. Mag. Nat. Hist., ser. 3, vol. I, 1858, 

 p. 248, pi. X, fig. 2; Geol. Surv. Canada, Org. Eem., Decade III, 1858, 

 p. 98, pi. ix, fig. 15), is more slender anteriorly than the variety inter- 

 media, and much more so than I. Ottawa. It measures : Length, 3|- 

 mm.; hinge-line, 2f mm.; height, 2 mm. (at the hinder moiety only). 

 Its surface is more or less punctate, and its marginal lip is neatly 

 pitted. The valves form a layer in the Birdseye Limestone, at the 

 White Horse Rapids, Isle Jesus. 



* Abundant on the bed-planes of a piece of thin-bedded limestone, from a loose block in 

 Sussex street, Ottawa, and probably belonging to thel upper part of the Chazy Formation (coll. 

 Mr. H. M. Ami). Quart. Jour. Geol. Soc. , vol. XLVI, 1890, p. 651. 



t Constituting, with a few bivalve molluscs, the greater part of an easily broken grey lime- 

 stone from Nepean, Ontario, belonging to the Chazy Formation (coll. Mr. H. M. Ami). Op. cit., 

 p. 551. 



X Abundant in a dark grey limestone of the Chazy Series (coll. Mr. T. W. E. Sowter). Op. cit., 

 p. 553). 



