170 STUDIES IN GEOLOGY, No. 3 



Super family MYACEA 



Family CORBULIDAE 



Genus CORBUI,A (Bruguiere) Lamarck 



Section ALOIDIS Megerle 



The Corbula lietis GROUP 



The two species of Corbula, section Aloidis, here described 

 from the Zorritos district, Corbula bradleyi Nelson, and 

 Corbula prenuncia n. sp., are members of a natural group 

 whose relationships are interesting in that they show a trans- 

 ition from somewhat acutely triangular, many-ribbed forms 

 to squarer, more coarsely ribbed forms. This series, with 

 the change in the characters named, is as follows : 



Corbula vieta Guppy 1 f , ,. ., 



_ , . , most and finest ribs 



Corbula heterogenia I most triangular J 



least inflated beaks 



Corbula bradleyi Nelson less triangular fewer and coarser ribs 



Corbula vieta Maury still less triangular still fewer and coarser 



(not Guppy) 197 ribs 



Corbula prenuncia Spie- most nearly square coarsest and fewest ribs, 



ker most inflated beaks 



136 These two are held distinct by Ball and are combined by Wood- 

 ring under the name of C. heterogenea Dall. Brief examination of 

 the Bowden material shows the two forms separated by Dall to 

 exist, and the matter rests on the specific validity of the characters 

 on which DaM made the distinction, namely, narrower, less elevated, 

 less prominent beaks, and the posterior part of the shell more pro- 

 duced in Dall's heterogenea than in his vieta. 



97 This form is one from the Santo Domingan Miocene, described 

 by Maury (Bull. Amer. Pal., vol. 5, No. 29, p. 231, pi. 39, fig. 13, 

 1917) as Corbula vieta Guppy, but to which species the shell figured 

 obviously does not belong, it being much squarer than the true 

 vieta, and having about half as many, much coarser ribs. Since 

 it is not possible to correct Maury's synonymy here, the form 

 described by her may be referred to as Maury's C. vieta. The 

 Santo Domingan form is probably a new species. 



