502 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



longitudinal, linear slit down the ventral side ; surface ornamented on the 

 ventral side by distinct vascular (?) markings, and having on the dorsal side 

 a broad flattened ridge ; phragmocone nacreous, and provided with a single 

 dorsal ridge and a ventral process, and often with a minute bulb at the apex.* 



The guards of this genus are mainly distinguished from those of Belem- 

 nites by their ventral slit and vascular markings. There were probably other 

 differences in the anterior termination of the phragmocone, as well as in the 

 structure of the animal, in regard to which nothing is known. 



There are some reasons for doubting whether or not d'Orbigny's name 

 Belemnilella can be properly retained for this genus. At one time, I was 

 inclined to believe that this type, instead of the similar group, might have 

 to inherit the older name Belemnites, because Lamarck, who was the first 

 regular binomial author that used it in a binomial sense as the name of a 

 genus, cited as his first examples figures of Breynius, that clearly represent 

 Belemnitella, and not the genus Belemnites, as now understood. He, how- 

 ever, also cites, in the same connection, figures of Klein's Tubulis marinis, 

 t. 8, figs. 2-13. I have not had an opportunity to consult the old work of 

 Klein's to which he refers; but my friend Mr. Gabb, after examining the 

 figures in a copy of it at Philadelphia, informs me that those cited by 

 Lamarck in part represent Belemnilella and in part forms belonging to the 

 genus Belemnites as restricted. Herrmannsen also cites Tubulis marinis, 

 Klein, as in part equivalent to Belemnites. Consequently, Lamarck had both 

 types included, and could not be regarded as restricting the name Belemnites 

 exclusively to either group. 



In 1826, Miller proposed (Trans. Geol. Soc Lond., II (2d ser.), page 

 63) the name Actinocamax for a genus which he supposed to differ from 

 Belemnites in having no alveolar cavity at the anterior end. It is evident, 

 however, from the figure of his type, that it is simply a guard broken off just 

 behind the termination of this cavity, and then with the broken end worn, as 

 has been suggested, probably during the life of the animal. Sowerby, Daniel 

 Sharpe. and Dr. Gray, maintain that Miller's type {A. verus, Miller) is really 

 neither more nor less than the B. mucronata, Schloth., the type of Belemni- 

 tella, d'Orbigny ; and this seems to be confirmed, at least so far as regards its 



* The phragmocone of this geuus is only known from natural casts of the alveolus, which show 

 that it was chambered as iu Belemnites. Whether or not it was produced in the form of a long anterior- 

 dorsal expansion, as in that genus, is, I believe, unknown. Thin lamina: of the phragmocone left iu the 

 ah cuius of some of our specimens show that it was pearly. 



