116 WHITBAVES ON CEETACEOUS AMMONITES 



A study of the whole of these specimeus has conviuced the writer that they should 

 be referred to the genus Desmoceras rather than to Placenticeras, and that they represent a 

 variety of D. a//itie in which the periodic arrests of growth in the septate portion of the 

 shell are either obliterated by lateral pressure or not developed. As the specific name 

 glabrum can scarcely be retained for the type of the species, whose surface is by no means 

 smooth, it may perhaps be used without impropriety for the less strongly ornamented 

 variety now under consideration. 



At first sight, the comparatively smooth specimens of this variety, such as that repre- 

 sented on Plate IX., are so dissimilar in aspect to the typical form of the species, as 

 illustrated on Plate VIII., that they might well be regarded as specifically distinct from 

 it. Some specimens, however, are intermediate in their characters. Thus, in an imperfect 

 shell of this species, from the Loon Eiver Shales of the Loon River, part of the outer 

 volution of which has been broken off in such a way as to expose a large portion of the 

 last volution but one, the inner of the two volutions has the character of the var. glabrum, 

 i. e., the absence of periodic growth arrests, while upon the outer one very distinct traces 

 of the distant and feebly developed transverse ribs characteristic of the typical form of 

 the species are preserved. 



Desmoceras Athabascense. (Sp. nov.) 

 Plate X. 



Shell very large, discoidal, compressed at the sides and narrowly rounded at the 

 periphery : volutions so closely coiled and so strongly embracing that the whole of the 

 inner ones are covered by the outer one, the umbilicus consisting of a very narrow 

 central depression or pit, on each side : chamber of habitation occupying between two- 

 thirds and three-fourths of the outer volution : aperture much higher than broad, almost 

 elliptical in outline but deeply emarginate by the encroachment of the preceding volution. 



Surface of the only specimen collected, which is not very well preserved, nearly 

 smooth and marked only with two or three distinct, broad but shallow and extremely 

 obscure radiating grooves or faint constrictions, on the outer portion of the chamber of 

 habitation. Sutures of the septa too imperfectly .preserved to be described with any 

 precision. 



Maximum diameter of the specimen, fifteen inches and a half : greatest breadth of 

 the same (at the aperture) five inches. 



Athabasca River, four miles below the mouth of the Pelican, in the lower 200 feet of 

 the La Biche Shales, associated with Acanthoceras Woolgari, — R. Gr. McConnell, 1890 : a 

 nearly perfect cast of the interior of the adult shell, with most of the nacreous or inner 

 layer of the test preserved upon the chamber of habitation. 



This fine species, which, so far as known, occurs at a horizon at least 500 feet higher 

 up in the Cretaceous than D. affine, appears to differ from that shell in its much larger 

 size, narrower umbilicus and somewhat different sculpture. Thus, the largest known 

 specimen of D. affine, in which nearly the whole of the chamber of habitation is preserved, 

 is about ten inches in its greatest diameter, whereas the maximum diameter of the type 

 and only known specimen of D. Alhabaxcense is fifteen inches and a half In D. affine a 



