PALEONTOLOGY ZORRITOS FORMATION 137 



narrow, and inflated, very slightly if at all prosogyrate, and 

 somewhat involute. Anterior surface rounded, gently con- 

 vex excepting a slight excavation just before the beaks; 

 median surface sharply convex but smoothly rounded, a 

 distinct rounded carina separating it from the posterior sur- 

 face, which is flat ventrally, and somewhat excavated 

 dorsally, though not markedly alate. Sculpture of about 

 21 ribs, broad and flat-topped anteriorly and narrower pos- 

 teriorly, with narrow, sharply channelled interspaces. Both 

 ribs and interspaces are crossed by microscopic lines of 

 growth which are stronger in the interspaces than on the 

 ribs. The ribs are more rounded near the beaks than ven- 

 trally, where they are distinctly flat-topped. The inner 

 surface of the shell is smooth except marginally, where there 

 is fluting. The raised elements of the fluting are slightly 

 sulcate centrally. Hinge not known. Length, 10.5 ; height, 

 14; diameter, 10 mm. 



Nelson's description of this species is inadequate, and he 

 gives no figure. The description here offered was made on 

 examination of his specimens together with an incomplete 

 right valve in the Hopkins collection. Nelson's specimens 

 are much worn, and have lost almost all of the epidermis ; in 

 this condition they appear different from the better pre- 

 served right valve, the under layers showing much narrower 

 ribs, but a small fragment of the original sculpture adher- 

 ing to one of Nelson's specimens enables their identification 

 with that in the Hopkins collection. All notes concerning 

 the sculpture are made from the latter form. 



The most closely similar of several forms obviously related 

 to C. affinis is C. sambaicum Maury, 152 from the Dominican 

 Republic. It is more nearly equilateral, being longer than 

 affinis, and is more broadly rounded in outline ventrally. 

 According to Maury the ribs of sambaiciim show definite 

 signs of having been universally noded ; those of affinis are 



Bull 



Amer. Pal., vol. 5, No. 29, p. 212, pi. 36, fig. 7, 1917. 



