2 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.65. 



Family NAUTILIDAE Owen. 

 Genus EUTREPHOCERAS Hyatt. 



According to Hyatt : * 



This genus includes these forms like the type E. dekai/i, which have globose 

 ananepionic substages, increasing subsequently with great rapidity in all their 

 diampters. The ana- and metanepionic substages are highly tachygenic and 

 these shells have very small, and often hardly perceptible and much flattened, 

 umbilical perforations. The siphuncles are subdorsan from the apex through 

 the nepionic stage in some species, in others this position is not maintained, 

 but the siphuncle is generally in the later stages near the dorsum and in the 

 ephebic stages it is dorsad of the center. 



The nepionic stage has longitudinal ridges and transverse bands, the former 

 disappearing in adults which are smooth. 



The form of the whorl in section is nephritic from an early age and changes 

 but little throughout life. 



The sutures are almost straight, having but slight ventral lobes, broad 

 ventrolateral saddles, lobes on the umbilical zones, and deep lobes in the zone 

 of impression. There are no annular lobes at any stage of development. 



Hyatt's remarks about the genotype are all based on specimens 

 from the Western Interior (" Dakotah ") , whereas the true E. dehayi 

 (Morton) is from New Jersey. It w^ould seem, therefore, that the 

 real genotype is the unnamed species mistakenly referred by Meek 

 and most other writers to E. dekayi. 



From the four other genera originally assigned to the family by 

 Hyatt Eutrefhocevas may be distinguished most easily as follows: 

 From Digonioceras Hyatt in having a nephritic rather than sub- 

 trigonal cross section of the whorl in the adult; from Cenoceras 

 Hyatt in having a nephritic rather than subquadragonal cross section 

 of the whorl in the adult; from Cymatoceras Hyatt in the lack of 

 broad ribs on the shell and in the straighter suture; from Nautilus 

 Linnaeus in the broad outline of all the later stages, the general posi- 

 tion of the siphuncle dorsad of the middle of the septum, and the 

 straighter sutures. (See fig. Ih.) From the genera Ilercoglossa 

 Conrad {Enclhnatoceras Hyatt) and Aturla Brown of the family 

 Clydonautilidae, which also occur in the Eocene, Eutrejyhoceras 

 differs sharply in suture. (See fig. la.) 



The writer has examined a number of specimens of Eutrephoceras 

 of Cretaceous age. The sutures are very much alike in all of them ; 

 the shell of all of the larger specimens, when preserved, is nearly 

 smooth ; and the position of the siphuncle is very much the same in 

 all. The conspicuous differences })etween them are in the form of the 

 cross section of the whorl and the size of the shell. These seem to 

 offer a valid basis for separation into species, and,, so far as the 

 writer's material goes, form and size are constant within considerable 



* Hyatt, Alpheus,, Phylogeny of an acquired characteristic: .\mer. Philos. Soc. Proc. 

 vol. 32, p. 555, 1894. 



