PTEROPODA. 159 
different countries of Europe and America. I shall have occasion farther to 
refer to this work. 
The position of these bodies in the animal kingdom remained for a long time 
undetermined, and they were arranged by most authors under the convenient 
head of incerta sedes. While it was easy to separate them from the Crinoidea 
(which they resemble in their annulated exterior), by their form and general 
characters, as well as by their intimate structure, it was more difficult to make 
any satisfactory reference to established groups. In 1845 they were referred 
by Mr. Austin to the Pteropoda.* This reference has generally been followed 
by later authors up to the present time, and is accepted by the writer as his 
conviction of their true relations. These bodies, however, in their compara¬ 
tively thick, calcareous test, and the annulating marks which affect the interior, 
and are visible upon the cast, are quite unlike the thin hyaline shells of most of 
the existing forms of Pteropoda. The interior casts of some of the Tentaculites 
present so many features in common with those of Cornulites that there is a 
very natural inference of a family relation between the two forms. At the 
same time, the free growth of the one and the attached mode of growth of the 
other (its usual condition in its young state at least), together with the 
difference in the structure of the test,f are sufficient grounds for a wide 
separation in a systematic arrangement of these genera4 
It may be necessary to state in this place that the reference of American 
species of Tentaculites to known European forms has not been sustained by 
critical examination. The Tentaculites ormtus of Sowerby seems to me to have 
its nearest representative in T. Niagarensis of the Niagara group. The T. tenuis 
is represented in the species of the Hamilton group; though a comparison of 
specimens may reveal a greater analogy with those of the Upper Helderberg 
formation. 
* Annals and Magazine of Natural History. 
f See illustrations of the structure of the test of Cornulites in Murchison's Silurian System, pi. 26. Also 
Twenty-eighth Report on the N. Y. State Museum of Natural History, 1875, plate 31. 
J Prof. McCoy has thus indicated the distinguishing- features of Tentaculites : “Their being- unattached, 
small size, and straight, regular form, separate them from the allied genus Cornulites ” ( Synopsis of British 
Pal. Foss., p. 63. 1855). 
