176 Trans. Acad. Sci. of St. Louis. 



commonly collected are right valves, the left valve 

 being rarely met with. The two valves are quite unlike, and 

 the left one, being less convex and having a less prominent 

 umbo, was described by White* as a distinct species. One 

 specimen in the White collection shows the two valves con- 

 joined, and although it is an imperfect distorted specimen it 

 shows, as has been pointed out by Winchell, f the relationship 

 of these two supposed species. The specimen illustrated is 

 one of those used by Winchell in his description of the genus 

 Dexiobia . 



Dexiobia halli Win 



PI. XV. f. 3-4. 



Original description. "Shell small, semi-elliptic; sub- 

 equilateral. Hinge line straight, extended ; in some specimens 

 as long as the greatest width of the shell. Right valve ex- 

 tremely ventricose, flattened and subalate toward the hinge 

 extremities ; left valve with a very small obtuse beak, and 

 slender posterior cartilage plate bearing a longitudinal median 

 furrow. Surface smooth." 



The dimensions of the most perfectly preserved of the type 

 specimens, a right valve, are, height 17 mm., length 19 mm., 

 and convexity 9 mm. 



Remarks. This species and D. lohitei were considered by 

 Winchell as the types of his genus Dexiobia. D. halli can 

 always be recognized by its smooth surface and its extended 

 hinge-line. Like D. whitei its right valve is most commonly 

 preserved, the left valve being rarely found. 



SCHIZODUS TRIGONALIS ( Win. ) . 



PI. XV. f. 21-22. 



Cardiomorpha trigonalis, Bull. U. S. G. S. loo: 169. 



Original description. " Shell of moderate size, triangular, 

 rather ventricose, with elevated, incurved beaks. Ventral 

 margin slightly convex anteriorly, slightly sinuate near the 

 posterior angle ; anterior angle regularly rounded to the sub- 

 truncate anterior side ; posterior angle rather acute, formed by 



* Proc. Bos. Soc Nat. Hist. 9 : 31. 

 tProc. Acad. Nat. Sci. Phil. 1863: 11 



