SYNOPSIS OF GENERA. xxxiii 



Sphenotus, n. g. [Typical species, Sanguinolites arcaeformis, Hall, and 



Cypricardia contracta, Hall.] 



Shell equivalve, very inequilateral, elongate sub-trapezoidal or cylindrical in 

 outline. Anterior end short. Posterior end usually obliquely truncate. Beaks 

 sub-anterior. Cardinal line long and nearly straight. Umbonal ridge defined, 

 extending from the beak to the post-inferior extremity. Post-cardinal slope 

 marked in all the characteristic species by a median ridge. Valves crossed 

 from the beak to the base by a more or less defined, broad cincture, which often 

 produces a constriction in the ventral margin. 



Surface marked by concentric striae of growth, and rarely by fine radiating 

 lines. In a single species {S. solenoides) the surface is further ornamented by 

 fine, irregular, elevated vascular lines on the posterior half of the shell, espe- 

 cially above the umbonal ridge. 



Hinge narrow, with two short, narrow cardinal teeth beneath the beak of the 

 right valve, and with one or two extremely slender lateral teeth. Ligament 

 external, contained in a slender groove along the cardinal line. Anterior mus- 

 cular scar strongly marked, situated close to the anterior margin. Posterior 

 scar shallow. Pallial line simple. 



The species united under the preceding generic description have been refer- 

 red to several genera by various authors. The genus Sanguinolites, by an 

 unwarrantable extension of its true characters and the disregard of the type, 

 has heretofore included many species of this group. Also, a few forms have 

 been placed in the genus Allorisma. They differ from both these genera in the 

 umbonal and post-cardinal ridges, their trapezoidal form, in the cincture cross- 

 ing the valves, and in the characters of the hinge. 



Examples: Sphenotus arcceformis, pi. Ixv, figs. 7-11. 



Sphenotus contractus, pi. Ixvi, figs. 1, 3-9, 11-13, 15 ; pi. xciv, fig. 2. 



Spathella, n. g. [Type, Spathella typica. Hall.] 



Shell equivalve, very inequilateral, wider behind, transversely sub-cylindrical. 



Anterior end short, narrowly rounded. Beaks sub-anterior, small. Umbonal 



slope rounded or sub-angular. 

 E 



