46 COLORADO COLLEGE STUDIES. 



With others, the writer formerly supposed that this shell 

 was a mere variety of G. pitcheri, but now fully confirms the 

 original opinion of Gabb and the revised opinion of Hill that 

 it is specifically distinct. While the Grayson Grypha}a often 

 more or less nearly approaches the so-called "Kiamitia 

 variety " of G. pitcheri, with respect to the elongated trian- 

 gular outline of the shell, it presents important differences, 

 such especially as the truncate anterior border, the sharp 

 angulation of the sulcus, and the very prominent and com- 

 pressed anterior and posterior folds of the left valve; and 

 these features are so constant under all the variation in shape 

 of the Grayson sholl that, not only for stratigraphic purj)oses 

 and 'convenience of expression, but even on the strictest bio- 

 logical grounds, this shell must be regarded as distinct from 

 G. pitcheri, while closely related to it.* Gabb's G. mucro- 

 nata and Conrad's G. navia are not considered by the writer 

 as synonymous, but the latter is regarded as merely an arched 

 and imbricated example of G. pitche^'i, lacking (as do all of 

 the stratigraphically low-occurring Gryphfieas of the Com- 

 anche) the compressed folds (especially the anterior com- 

 pressed fold) ancl truncate anterior border which are the 

 fundamental aracteristics of the left valve of G. mucronata. 



At u.j above-given typical locality of the yellow marl 

 phase of the Grayson, there is, in the lower part of the marl, 

 a horizon of profuse occurrence of typical Exogyra arietina 

 and, just below this, one of Turrilites brazosensis and Hol- 

 aster completus. In the upi^er part of the marl at the same 

 locality, the Exogyrd arietina is small and not abundant, the 

 Turrilites plentiful; hniiYie Holaster compJet as has not been 

 obtained, and, if it occurs there, must be rare.f 



The fauna of the Grayson marl presents some affinities 

 to that of the Choctaw limestone, but far more is peculiar to 

 either fauna than is common to both. 



♦ strong incurvature of the beaks and the conspicuous and intervalled margins 

 of imbrication are also constant or nearly constant features of G. mucronata, but as 

 tlieso are features exhibited by some examples of G. pitcheri, they are not of equal 

 diagnostic value with the characteristics above mentioned. 



t It is to be noted, however, that the latter species, like Hola.iter si'm/'/ex fre- 

 quently occurs in groups, so that what has appeared rare or absent in a horizon 

 may, by some fortunate find, become locally abundant. 



