82 CARBONIFEROUS AMMONOIDS OF AMP:RICA. 



crenulatious seen on most other species of this group. Constrictions deeply 

 incised, four to a revolution, bending sharply forward on the abdomen. 



Occiirrence. — Lower Carboniferous, St. Louis stage, Crab Orchard, 

 Ky., and the same horizon in the so-called Fayetteville shale of Batesville, 

 Ark. The figured specimens came from the Arkansas locality, and are 

 deposited in the geologic collection of Leland Stanford Junior University, 



California. 



Genus Gastrioceras Hyatt. 



This genus was originally established by Hyatt" to include evolute 

 species with open umbilicus, trapezoidal or semilunular cross section, 

 and usually ribs or tubercles on the sides; the species included liy Hyatt 

 in this genus all have prominent siphonal saddles, first lateral saddle broadly 

 rouuded, second lateral saddle broad, but inclined to be pointed: tlie 

 siphonal lobes are long, narrow, and pointed, and the lateral lobes broad 

 and pointed. In all the species cited by H^-att'' as belonging to Gastrio- 

 ceras, there is but a single pair of lateral lobes visible — that is, on tlie 

 sides of the shell; and Hyatt '' limits Gastrioceras to forms with a single 

 pair of lateral lobes and with the second pair on tlie innbilical shoulders. 

 Hyatt" refers G. russiense Tzwetaev to his genus Parakgoceras, because that 

 species has the second pair of lateral lobes on the sides of the shell and not 

 on the umbilical shoulders. But Gastrioceras russiense has just the same 

 number of lobes as all other known species of Gastrioceras, namel}', nine in 

 all, and lacks the lobe on the umbilical border, which is characteristic of 

 Paralpfioccras. Dr. K. von Zittel' confines Gastrioceras to forms with a 

 single pair of lateral lobes. But the relations of Gastrioceras, Ghjphioceras, 

 Goniatites, and Parakgoceras have been best worked out by Karpinsky, who 

 shows that there is no marked distinction between Goniatites and Gastrio- 

 ceras; that both have the same number of lobes and saddles — nine of each; 

 that the second pair of lateral lobes may be on the umbilical shoulders or 

 on the sides of the shell, thus differing from Paralegoceras, in whicli the 

 third pair of lateral lobes is on the umbilical shoulders. Gastrioceras usually 

 has a trapezoidal cross section and umbilical ribs; but some species lack the 



« Proc. Boston Soc. Nat. Hist., Vol. XXII, p. 327. 



" Ibid. 



•^Second Ann. Rept. Geol. Surv. Texas, p. 355. 



<abid. 



'Grundziige der Palwont., )). 399. 



