190 University of Texas Bulletin 



saddles of the suture, but the saddles are much more slender in our 

 species and the lobes are narrower. The number of the saddles and 

 lobes is probably the same. In its external form our species distin- 

 guishes itself from P. Cumminsi by a still wider umbilicus, more 

 flattened flanks and a much higher whorl. But both species are en- 

 tirely distinct from the younger species of the genus. This refers 

 especially to the form and number of branches on the siphonal saddle, 

 which in the older forms is moich simpler than in the younger ones, 

 as we shall see even in the description of the new species found by 

 Beede in the Clear Fork beds. Much more complicated are all those 

 species described from the Double Mountain beds {P. Hilli Smith, P. 

 n. sp. of Ouanah) and of the Leonard formation {P. compressus, P. 

 vidriensis) . There does not seem to exist any doubt that our older 

 forms, the one described here and P. Cumm,insi, are the oldest ante- 

 cessors known of the whole tribe of Perrinites. It should be men- 

 tioned also, that in the younger forms cited above, the branches of the 

 siphonal lobe are much less asymmetrical and much deeper than in 

 the older ones. 



In general we can say that Perrinites is not a genus which may 

 easily serve to distinguish horizons as all the species which belong to 

 it are very similar to each other. The external form differs little, and 

 the lobes have always the same general form. The genus lives through 

 a relatively great number of quite different stratigraphical and pale- 

 ontological horizons without changing much. Still, with care, it is 

 possible to distinguish the different species quite clearly; but a deter- 

 mination of age should not be based entirely on a single specimen of 

 Perrinites, although it might help in combination with other am- 

 monoids. 



At the locality four miles south of Dundee, our Perrinites is by far 

 the most frequent form, being represented in our collection by twenty- 

 one specimens. Nearly everywhere Perrinites has been found it is 

 much more frequent than any other ammonoid genus. 



Stacheoceras n. sp. 



Shell very involute, slightly flattened on the flanks, well rounded on 

 the venter. Cross-section of the whorl elliptical, nearly as wide as 

 high, greatest width about one-third above the umbilical border. 



