castalina i205 



Kky to species of Castalina. 



Shell with strong posterior folds. C. undosa. 



Smooth or but feebly plicate behind. 



Almost strictly rhomboidal. C. orbignyi. 



Subrhomboid or subtriangular. 



With strong, elongated umbonal ridges, C. psammoica. 

 Umbonal ridges short. 



Post-dorsal groove well developed. C. martensi. 



Post-dorsal groove scarcely developed. C. nehringi. 



Group of Castalina martensi. 

 Characters as in the genus. 



Castalina martensi von Ihering. 



Shell irregularly rhomboid, slightly inflated, having the disks 

 a little flattened, solid, inequilateral ; beaks moderately full and 

 elevated, their sculpture consisting of a few wide, rather feeble, 

 radial ridges, separated by narrow grooves, these ridges do not 

 extend over the disk ; posterior ridges well developed, narrowly 

 rounded ; above it is a deep, wide, radial groove, which usually 

 ends in a slight sinus on the border of the dorsal slope ; anterior 

 end somewhat narrowed, rounded; base line curved; dorsal 

 slope oblic[uely truncate, there being a rounded point at the 

 hinder end of the posterior ridge; surface irregularly, concen- 

 trically sculptured, there sometimes being a few, faint wrinkles 

 on the dorsal slope : epidermis thick, brown, rather dull ; 

 pseudocardinals radial, split up into three or four divisions in 

 each valve, but slightly roughened; laterals double in the left 

 valve, single in the right, slightly vertically or sometimes ob- 

 liquely striate ; nacre dirty whitish or brownish-purple, thick- 

 ened in front ; beak cavities deep ; anterior scars impressed. 



Length y2)' height 53, diam. 32- mm. 



Southern Brazil. 

 Castalina martensi von Ihering. Zool. Anz., XI\\ 1891, p. 

 . 477; Arch, fiir Nat., 1893, P- 81, pi. iii. fig. 5. — Simpson. 



Syn., iqoo, p. 865. 



The shell is rather more elongated and rhomboid than either 

 of the allied forms. 



