128 STUDIES IN GEOLOGY, No. 3 



from the Helvetian to the Astian of Italy, is somewhat sim- 

 ilar in sculpture, and the variety helvetica Mayer, 135 from 

 the Helvetian to the Astian, is similar in shape. The only 

 closely related American form is A. gabbi Pilsbry and John- 

 son, 136 from Santo Domingo. It is, however, more delicate, 

 not as much inflated,, and has more distinct primary radial 

 ribs, with occasional strong interstitial ribs which are not 

 present in berryi. A gabbi lacks the imbrication of growth- 

 lamellae common to the ventral slope of berryi. The two are 

 very close in type of sculpture. 



Upper Zorritos ; Variegated (basal). Quebradas Zapotal 

 and del Grillo. 



Order TELEODESMACEA 



Superfamily ASTARTACEA 



Family CRASS ATELUTIDAE 



Genus CRASSATEUJTES Kriiger 



Section SCAMBULA Conrad 



Crassatellites (Scambula) nelsoni (Grzybowski) 



PLATE VII Fig. 8. 



Crassatella gibbosa Nelson, Trans. Conn. Acad., vol. 2, p. 203, 

 pi. 7, 'fig. 9, 1870 (ex parte) ; not Sowerby. 



Venus nelsoni Grzybowski, Neues Jahrb., Beil. Bd. 12, p. 639, pi. 

 19, figs. 2, 2a, 1899. 



Mature shell heavy, large, subtriangular, inflated; young 

 shell more inequilateral. Anterior margin rounded; ventral 

 margin arcuate in mature shell, the curve more shallow in 

 adolescent specimens. Posterior margin biangular, truncate 

 in adult, more produced in adolescent shell. Umbones high, 

 full, distinctly flattened above, the flat surface sloping back- 



133 Ibid.; p. 37, pi. 10, fig. 39. 



l3 *Proc. Ac. Nat. Sc. Phila., vol. 69, p. 193, 1917. 



