I 200 TKTRAPLODON 



H. and A. Adams figure a young shell and credit the species 

 to Humphry, but do not describe it. 



The description by Sovverby of this form is inadequate and 

 the figure shows a shell mucli resembling T. quadrilaterus. It 

 may, however, be distinct. It seems to be more strongly, con- 

 centrically sculptured and to have shorter, narrower, more nu- 

 merous umbonal ridges, which end higher uj) on the shell, and 

 the inner ones coalesce below, while in quadrilaterus they do 

 not. 



Tetrapiodon ovATiis (Sowenby). 



Shell subquadrately triangular, solid, inequilateral ; beaks 

 full and high, rather sharp, having a few, strong, widely 

 spaced radial ribs, which extend about half way over the disk 

 and suddenly end at one of the growth lines ; anterior end cut 

 away above, rounded below ; base line curved ; outline of dorsal 

 slope oblique, elevated to a decided angle in the middle ; pos- 

 terior ridge full, sharp above, ending in a blunt, point near the 

 base of the shell, the outline above this point being slightly 

 incurved ; epidermis pale olive with concentric, darker bands. 



Length 66, height 52 mm. 



Brazil. 

 Castalia ovata Sowerby, Conch. Icon., XVII, 1869, pi. i, fig. 4. 

 Tetrapiodon oz'atits Simpson, Syn.. irjoo, p. 864. 



Sowerby never gave any measurements or descriptions of 

 the interior of his Naiades and for that reason I am unable to 

 fully compare this with quadrilaterus, to which it seems closely 

 allied. The radial bars are wider and more widely spaced than 

 in quadrilaterus, they end suddenly and evenly near the middle 

 of the disk, the color is lighter and is banded. 



Tetrapi.odon rhtusus (Hupe). 



Shell almost regularly triangular, inflated, solid, somewhat 

 inequilateral, with a short ligament ; beaks very high, full, 

 sculptured with a few, wide, radial ridges separated by nar- 

 rower grooves ; the ridges do not extend half way over the 

 disk; the rest of the shell is rudely and irregularly concentric- 

 ally sculptured; posterior ridge high, -narrowly rounded, ending 



