I/O 



IStudies IX Geology, No. 3 



Superfamily Myacea 



Family CORBULIDAE 



Genus Cori;ui,a (Bruguiere) Lamarck 



Section Aloidis ]\Iegerle 



The Corbida zietis Group 



The two species of Corhiila, section Aloidis, here described 

 from the Zorritos district, Corbula bradleyi Nelson, and 

 Corhnla prcinnicia 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 

 Corbula heterogenia 

 Dall 106 



Corbula bradleyi Nelson 



Corbula vieta Maury 

 (not Guppy )^" 



Corbula prenuncia Spie- 

 ker 



most triangular 



less triangular 

 still less triangular 



most nead^y square 



most and finest ribs 

 least inflated beaks 



fewer and cparser ribs 



still fewer and coarser 

 ribs 



coarsest and fewest ribs, 

 most inflated beaks 



*'**' These two are held distinct by Dall and are combined by Wood- 

 ring under the name of C. hetcrogenca 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 Dall made the distinction, namely, narrower, less elevated, 

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

 duced in Dall's hetcrogenca than in his vietii. 



''"This iorr\ 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. 



