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, annotated 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 

 striae, rarely by longitudinal striae. 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. 



