172 Studies ix Geology, No. 3 



It is a species noted for its unusual size, and will undoubt- 

 edly prove to be one of the characteristic fossils of the 

 Zorritos group. 



Discussion of its affinities is hardly necessary after the 

 indications ofifered in the brief consideration of the Corbu- 

 la vieta group. It is of a type characteristic of Antillean 

 Miocene horizons, and its presence in the Zorritos offers 

 further evidence of the close relationship between the mollus- 

 can faunas of the two regions during Lower Miocene time. 



Corbula (Aloidis) prcnuncia n. sp. 

 PivATE X — Fig. 12. 



Shell small, inflated, triangular. General form pyramidal; 

 anterior margin short, rounded ; ventral margin elliptical ; 

 posterior margin biangular, very little produced. Umbone of 

 right valve high, prominent, strongly incurved, inflated. 

 General surface smoothly round ; rostrum keels well marked, 

 but posterior portion not markedly gibbous. Sculpture of 

 heavy, proportionally coarse, concentric lirae, about 18-20 

 on the right valve. Inner shell and left valve unknown. 

 Length, 5.5 ; height, 5.5 mm. 



This species, although somewhat imperfectly known, offers 

 sufficient evidence of a distinctive character. It is related to 

 the forms mentioned in the presentation as a whole of the 

 Corbula vieta group. It is close to Corbula bradlcyi Nelson, 

 and was thought at first to be the young of that species, but 

 a comparative study of the young of other species in the 

 section Aloidis shows that it is distinct as noted in the dis- 

 cussion of relationships. 



Lower Zorritos. North of Quebrada Boca Pan. 



Section Cuneocorbula Cossman 



Corbula fCuncocorbula) fabiforiiiis n. sp. 



Plate X — Fig. 15. 



Shell of average size, ovate, somewhat produced. Ante- 

 rior margin rounded, ventral margin elliptical ; posterior 



