I04 University of Texas Bulletin 



tion in the middle, by the presence of a second lateral lobe on the flank, 

 and by the strongly scalloped saddles, which are rounded only at the 

 upper end. The suture of the typical Thalassoceras is much more 

 developed than that of Prothalassoceras, which is easily explained by 

 their different age. 



Judging by the suture alone, Th. Gemmellaroi Kain. should be in- 

 cluded in our new genus, but as there is only a very small specimen 

 known, the generic position of that species will remain to a certain 

 degree doubtful until the examination of a larger one will show if 

 the suture develops to that of the Sicilian Thalassoceras or retains the 

 outline figured by Karpinsky; which latter case I think to be the more 

 probable. 



Prothalassoceras has been found only at the base of our Hess forma- 

 tion, a horizon which has yielded but one other cephalopod until now, 

 and which in general contains only a great number of Fusulinidae 

 and a few brachiopods. The bed where the new genus was found is 

 far below those strata which correspond in age to the Sosio beds of 

 Sicily. 



Prothalassoceras Welleri nov. sp. 

 PI. V, Fig. 14-18 



Shell discoidal, extremely involute, with compressed flanks and 

 rounded venter, but without a development of ventral shoulders ; cross- 

 section more or less parabolical, much higher than broad, the greatest 

 width being found in the umbilical region. Umbilicus practically 

 closed, no umbilical shoulder developed, the flank bending gradually 

 down towards the umbilicus. The only ornamentation which could 

 be discovered is found on some of the shell preserved on the flank near 

 the umbilical region and consists of fine lines that from the umbilicus 

 curve strongly backward ; on the upper part of the flank and on- the 

 venter the shell is not preserved and nothing can be said about the fur- 

 ther course of those lines. The body chamber is only in part known; 

 what is preserved of it has the length of about one-half of the last 

 whorl. 



The septa are well separated from each other and the suture is 

 straight between the sipho and the umbilicus; it is extremely simple 

 (pi. V, fig. 17, 18), being composed of the siphonal lobe, a lateral one 



