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 ineerta 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 genera.f 



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 ornatus 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. 



* A nnals and Magazine of Natural History. 



t See illustrations of the structure of the test of Cornulites in Murchison's Silurian System, pi. 26. Also 

 Twenty-eighth Report on the N. T. State Museum of Natural HUiory, 1875, plate 31. 



\ Prof McCoy has thus indicated the distinguishing features of Tbntacclitks : "Their being- unattached, 

 small size, and straight, regular form, separate them from the allied genus Cornulites" (Synopsis of British 

 Pal. Foss., p. 63. UBB). 



