215 



resembles the 0. planoconvexum of Hall, from the Trenton limestone of 

 Wisconsin, in its planoconvex transverse section and in the close approxi- 

 mation of its septa, but differs therefrom in its much less rapid increase 

 in thickness, less compressed sides, and, more particularly, in the entirely 

 different position of its siphuncle, which latter, in 0. planoconvexum, is 

 said to be "centrally situated on the flattened side." 



AscocERAs COSTULATOM, Whiteaves. 



Plate 22, fig. 1. 

 Ascoceras costulatum, Whiteaves 1896. Oanad. Reo. Sc, vol. VI., p. 394. 



" Shell large, elliptio-subovate, longer than broad, and broadest in 

 advance of the midlength, the neck or anterior prolongation of the body 

 chamber being broken off in the only specimen known to the writer ; 

 outline of transverse section in the broadest part apparently elliptical, 

 the dorsum and venter being compressed and the sides slightly expanded. 



"Surface transversely but rather finely ribbed, the ribs averaging from 

 seven to nine in the length of one centimetre, and rather closer together 

 near the aperture than in the more expanded portion. 



" Sigmoidal septa apparently three in number, though their distances 

 apart on the dorsum cannot be ascertained. The suture, however, which 

 forms the line of demarcation between the decurrent extremity of the 

 body chamber and the septate portion, on both sides, is clearly defined. 

 It shows that tlie body chamber extends as far backward as to within 

 about half an inch from the blunted pointed posterior end, that it is 

 dilated or produced laterally, toward the dorsum, for a short distance 

 posteriorly, and concavely constricted for a much longer distance ante- 

 riorly." 



Little Black Island, Lake Winnipeg, D. B. Dowling and L. M. Lambe, 

 1890 : a badly preserved cast of the interior of the shell, with one side 

 much worn, but with portions of the test preserved on both the venter 

 and dorsum. 



"This species bears some resemblance to A. Boheniicum of Barrande, 

 particularly in size and in the general style of its surface markings. The 

 ribs or riblets of this Bohemian species, however, are represented as finer 

 and very much more numerous. Thus, according to Lindstrom, in A. 

 Bohendcum there are as many as twenty-two riblets in a length of five 

 millimetres, and hence, presumably, forty-four to a centimetre, but in 

 the present species there are only from seven to nine ribs to a cemi- 

 metre. Among Canadian species, A. costulatum would seem to be 

 nearest to A. Canadense, Billings, the type of Hyatt's genus Billingsites, 

 and hence may be referable to that genus. The surface markings of A. 

 Canaden-ie, however, are still unknown, or at least not preserved in any 

 of the specimens in the Museum of the Geological Survey." 



