176 Silurian Fossils of Canada. 



dorsal side are full half the distance between them nearer the 

 aperture than they are on the ventral side. 



The probable length of this Orthoceras judging from seve- 

 ral imperfect specimens which I believe to belong to the species is 

 from one foot and a half to two feet. 



The surface exhibits some indistinct flat longitudinal ridges, 

 each about one third of a line wide and half a line distant from 

 each other. 



Locality and Formation. — Pauquettes Kapids, Ottawa River, 

 Black River limestone. 



Collector. — Sir W. E. Logan. 



Orthoceras rapax. N. s. 



Description. — Large, section circular, septa distant a little less 

 than one fourth of an inch where the diameter is between five and 

 six inches. Siphuncle large, marginal very nearly in contact with 

 the shell on the ventral side. The rate of tapering appears to be 

 about one inch and a half to the foot. The depth of the chamber 

 of habitation is about one half greater than the width of the aper- 

 ture. 



Of this species we have portions of two individuals. One is a 

 fragment of the oral extremity entirely deprived of the shell, and 

 exhibiting a good cast of the interior of the chamber of habitation. 

 The following are the dimensions. Length fourteen inches; 

 diameter at aperture six and a half inches ; at fourteen inches from 

 aperture, four and three fourths inches ; diameter of siphuncle one 

 inch and three fourths at the broken or smaller end of the speci- 

 men ; the first twenty one septa occupy a length of four inches 

 and seven eighths. 



The second specimen is fifteen inches and a half in length, 

 and tapers from six inches to four as nearly as can be determined. 

 In the first five inches of the smaller extremity there are twenty 

 septa ; in the next inch five ; in the next inch and a half twelve, 

 and in the remainder from four to seven in the inch. 



The distance of the septa is thus variable in the same specimen, 

 but judging from the general appearance of the two examined, my 

 present impression is that the average distance must be from two 

 and a half lines to three lines, or a little less than one fourth of an 

 inch at a diameter of from four to six inches. The proportional 

 distance of the septa to the diameter of the shell therefore 

 would be about as twenty to one. 



