i66 Studies in Geology, No. 3 



lated." These characters are exactly as noted, but in the 

 group he set aside as M. zorritensis as well as the other are 

 specimens which show an even gradation from the longer, 

 posteriorly more pointed, less inflated forms typical of his 

 "species indeterminate" to the shorter, generally rounder 

 forms he called zorritensis, and a close study of all char- 

 acters shows the relationship to be sufihciently close to 

 warrant their grouping into a single species. The species 

 is simply variable, as is the case in many other natural 

 specific groups in the genus. 



M . corrifensis shows remarkable similarity throughout to 

 M. lateralis Say,^"^ from the Miocene to Recent of the 

 Atlantic States. Such differences as occur are functional of 

 the greater thinness and fragility of sorritensis, whose hinge 

 armatvire is more delicate, the lateral laminae in the right 

 valve being far less stout than in lateralis; the cardinal 

 teeth, however, are entirely similar. Although every char- 

 acter of zorritensis has its analogue in lateralis, even the 

 somewhat wide variations of the northern species being 

 paralleled by similar mutations in zorrite^isis, the complete 

 difference in weight gives rise to a contrast which deserves 

 systematic recognition. 



Zorritos formation. Zorritos. 



Subfamily Pteropsidinae 



Genus Labiosa (Schmidt) Moller 



Subgenus Raeta Gray 



Labiosa (Raeta) gabbi Pilsbry and Johnson 



Racta canaliculata Gabb, Trans. Amer. Philos. Soc, vol. 15, p. 248, 

 1873; (not Say, 1821) . 



Labiosa (Raeta) gabbi Pilsbry and Johnson, Proc. Ac. Xat. Sc. 

 Phila., vol. 69, p. 202, 1917. 



"The shell is oval-triangular, the beaks nearer the anterior 



'^' Md. Geol. Surv., Pliocene and Pleistocene, p. 194, pi. 55, figs. 

 1-4, 1906. 



