PTEROPODA. 165 
forms of the genus appear to be quite free in their mode of growth (at least in 
their advanced stage), and in this respect resemble Tentaculites. 
In geological distribution, we find the species of Cornulites restricted to 
strata of Silurian age, beginning in the Trenton limestone, and represented by 
species in the Hudson River, Clinton and Niagara groups. Thus far, none 
have been described from strata of Devonian age, and none are known to me 
as occurring at a later period than the Niagara. On the other hand, species 
of the genus Tentaculites have been described from all the formations from 
the Trenton limestone to the Upper Devonian, inclusive. 
As the result of studies made on the characters of typical forms of the genus, 
I propose to recognize as Tentaculites only those organisms which consist of 
straight, elongate, attenuately conical tubes. These tubular bodies are, for the 
most part, annulated by abruptly elevated rings, closely arranged near the apex 
or point of origin, and becoming more distant and stronger with the increasing 
size of the shell and their distance from the apex. The surface between the 
annulations, and often the annulations themselves, are marked by fine transverse 
striee, rarely by longitudinal strim. More rarely, also, there are forms which 
are not marked by annulations, but have otherwise the characters described. 
The annulations are sometimes irregularly disposed, as in T. gyracanthus, 
where this character extends to all parts of the individual; while in others this 
feature is confined to certain portions, as near the aperture. The section of 
the tube is always circular when not accidentally compressed. The cavity 
forms a single chamber without septa, but the annulations of the exterior are 
marked upon the interior by constrictions of the tube. The cavity does not 
extend to the apex—that part being partially filled w ith a calcareous deposit 
formed by the tissues during the growth of the animal. This deposit is 
variable in its extent in individuals of the same species, and extremely different 
in degree in different species of the genus, sometimes filling a considerable 
portion of the entire length. The open extremity or mouth of the tube is 
directly transverse, without indication of anterior or posterior sides. The 
notch or apparent slit which often occurs in flattened specimens is due to 
compression or exfoliation. 
