TERTIARY FOSSILS. 63 



of the Geological Survey of the State to help me, I was four years before I could 

 obtain a single specimen. 



I am convinced that Tamiosoma is a genus of Hippurilidce, for the following 

 reasons : 



1st. The form of the shell, the septate lower portion of its cavity, the longitudi- 

 nally tubular walls, and the chamber at the top, for the reception of the animal, 

 are characters which are found combined only in this family. 



2d. Its mode of growth is exactly analogous to that of the genera Hippurites 

 and Radiolites. 



3d. The structure of its walls is identical with that of Radiolites; differing only 

 in the relative thickness, as compared with the size of the interior cavity. This 

 analogy is carried out even to the extent of the prisms or columns, which consti- 

 tute the greater bulk of the walls, and are longitudinal and normally hollow, in- 

 stead of being transverse and solid as in Inoceramus and Pinna. 



4th. The septate arrangement of the lower part of the interior cavity is analo- 

 gous to that existing in Radiolites, as demonstrated by Woodward in the case of 

 R. Mortoni; the septa in that species differing only in being much less numerous, 

 and a little more symmetrical than in our fossil. (See "Woodward, Quart. Jour. 

 Geol. Soc, Lond., Vol. XI, pi. 5, fig. 2.) 



The only arguments that I can find against my position are : 1st. This shell is 

 Tertiary, and the Hippuritidoz are all Cretaceous. Although this would be doubt- 

 less the first objection urged, I can only see in it a reason against a priori argu- 

 ments on negative grounds, too common in science. Because the family has never 

 heretofore been found outside of the Cretaceous formation, is no reason why its 

 vertical range may not be extended by discoveries now or hereafter. 



2d. The thinness of the walls of the shell, as compared with Radiolites, which 

 I have called its nearest Klly. 



So long as the details of the structure are preserved, this cannot be a valid ob- 

 jection. 



3d. No trace has yet been found of ligament insertions, nor of the enormous 

 hinge teeth characteristic of the family. But, as yet, no specimen has been found 

 showing the entire shell, and the thinness of the walls warrants us, by analogy, in 

 predicating a delicate hinge when that portion shall be discovered. 



