96 NEW YORK STATE MUSEUM 



attained by the species of these genera, they appear to us as a group of 

 dwarfed forms comparable in their entirety to the small specimens with 

 constricted apertures among later (mesozoic) cephalopods considered as 

 dwarfs by Pompeckj. 



One specimen, consisting of a badly crushed living chamber which may 

 belong to this species, was found in one of the geodes of the upper Guelph 

 at Shelby, and another was obtained at Rochester. The latter is probably 

 also somewhat crushed, for its section is very traverse, the dorsoventral 

 diameter of the body chamber being one third less than the transverse 

 diameter. This specimen measures 50 mm on the ventral curve and has an 

 apertural transverse diameter of 20 mm and an apical diameter of 8 mm. 



It is a noteworthy fact that, with the exception of one or two 

 specimens, all have retained only the living chamber and a few camerae, the 

 apical parts being gone. It is therefore quite probable that this organism 

 was in the habit of discarding from time to time some of its oldest camerae. 



There are no forms in the Guelph or Niagara similar enough to invite 

 comparison with this species or to necessitate distinctive characterization, 

 but Cyrtoceras clitus Billings 1 should be cited here as possessing 

 similar size, like curvature and contraction of the aperture, though still 

 differing in the amount of expansion, this being much more rapid in the 

 apical part of Pot. sauridens. 



J. W. Spencer has described and figured 2 the cast of a living chamber 

 as Cyrtoceras reversum. This cast resembles so closely that of the 

 same part of P. sauridens that it doubtless belongs to that species. 

 The author has succeeded however in making a very remarkable fossil out 

 of this fragment, viewing it in a wrong direction and then calling it 

 "reversum." He states : " Its form is rapidly tapering with a considerable 

 curvature, until it ends in a rounded point." This rounded point is, how- 

 ever, the aperture, and the tapering is that of the living chamber toward 

 the aperture. From this misconception arose the other, that " the convex 



'Cat. Silurian Fossils of the Island of Anticosti. 1866. p. 85. 

 2 Mus. Univ. Missouri. Bui. 1884. p. 60, pi. 7, fig. 8. 



