166 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 sorritensis, and a close study of all char- 

 acters shows the relationship to be sufficiently 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. zorritensis shows remarkable similarity throughout to 

 M. lateralis Say, 191 from the Miocene to Recent of the 

 Atlantic States. Such differences as occur are functional of 

 the greater thinness and fragility of zorritensis, whose hinge 

 armature 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 zorritensis, 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 



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

 1873; (not Say, 1821). 



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

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



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



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

 1-4, 1906. 



