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 tbey 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. Sterlingensis* and also the T. incurvus of Shumard. 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 striaa, and it is fair to 

 presume that the other specimens have been similarly marked. The T. 

 Richmondensis is smaller than the figures of T. Oswegoensisf 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. 



t This species has been compared with curved individuals of T. elimgatus, 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. 



