PTEROPODA. 
163 
A critical examination of all the forms at present referred to the genus 
Tentaculites suggests an inquiry as to the reliable characteristics of the 
genus. The inquiry is the more pertinent in its special reference to the 
recognition, as veritable Tentaculites, of the Lower Silurian species of the 
preceding table. 
An examination of the figures and descriptions which have been given of 
these four species of Tentaculites shows that they are all of curved forms, and 
that they possess characteristic and peculiar features in their annulations which 
are quite distinct from any of the Upper Silurian or Devonian species of this 
country, with the exception of T. distans of the Niagara group, which is 
probably a Cornulites. They are moreover, where well preserved, always 
longitudinally striated, a feature unknown in authentic species of the Upper 
Silurian and Devonian formations, except in a single form. 
It seems scarcely possible that such external differences can be simply 
those marking the geological age of the species; and I infer that these 
characters have a deeper significance. Since the preceding pages were in type 
I have had an opportunity of examining specimens of the Lower Silurian forms, 
T. Richmondensis and T. Sterling ensis* and also the T. incurvus of Siiumard. The 
two first named preserve all the characters above described, while the last (as 
seen in a block containing four individuals of the curved forms) is too much 
worn for satisfactory generic determination. At the same time there is a 
single, almost straight individual on the same stone, which is marked by 
the characteristic annulations and distinct longitudinal striae, and it is fair to 
presume that the other specimens have been similarly marked. The T. 
Richmondensis is smaller than the figures of T. Oswegoensis f of Meek and 
Worthen, but otherwise very similar in character, and the proportionate 
number of annulations agreeing very nearly with the description of the 
latter species. The specimens of T. Sterlingensis, communicated as above, are 
of smaller size, corresponding with the figures of that species, as cited. They 
* Through the favor of Mr. S. A. Miller of Cincinnati. 
f This species has been compared with curved individuals of T. elongatus, to which it has only a very 
remote resemblance; and T. elongatus is never curved except through accident, so far as known to the 
writer. 
