MUSEUM VF COMPARATIVE ZOOLOGY. 309 



truncation about as much as the posterior cardinal margin falls toward the 

 upper angle of the same; truncation vertical, but hardly angulated; a faint 

 ridge running from the beaks to the upper corner, more marked in the smaller 

 valve ; sculpture of fine not very regular concentric undulations, coinciding 

 with the lines of growth; surface finely granular, but appearing nearly smooth; 

 interior polished ; the sinus deep and rounded ; margins very thin, smooth, and 

 plain; not interrupted under the beaks; ossicle bridge-shaped, wide, short, 

 concave behind in the middle line, very large for the size of the shell ; points 

 of insertion not perceptibly raised; extreme length of shell 6.25; length of 

 anterior part 4.75; max. alt. 6.0; diameter 2.75 mm., of which the deeper 

 valve takes about 1.50 mm. 



Habitat. West of Florida in 17 fms., one specimen; Marco, Florida, in 2 

 fms., H. Hemphill. 



This unpretending little shell has the aspect of a very young Periploma or 

 Thracia. It is only when the interior is examined that its peculiarities become 

 manifest. It is possible that, among the innumerable fossil genera or sections 

 which have been proposed, some one may eventually turn out to include the 

 present form, but none of those I have been able to examine agree with it. 

 Gorimya Agassiz is perhaps the nearest, but has submedian beaks with one or 

 two internal ribs, the posterior cardinal margin slightly thickened as if for an 

 external ligament, and there is no evidence of a pallial sinus, or rather the po- 

 sition of the posterior adductor scar would indicate that there was no sinus. 



The Californian species differs from the Floridian in having the beaks less 

 posterior, and, though a larger shell, in having a more slender and delicate 

 ossicle, which resembles, in some sort, a very long- winged butterfly with its 

 wings spread. 



It is interesting to add to the links connecting the East and West American 

 faunae, and pleasant in so doing to be able to clear up the obscurity which has 

 somewhat interfered with the relegation of this genus to its proper place in the 

 system. 



Subgenus BUSHIA Dall. 



Shell inequi valve, inequilateral, truncated behind, but not gaping; porcel- 

 lanous; concentrically sculptured; hinge toothless, with a large U-shaped 

 ossiculum fitting in the apices of the beaks, which are filled with solid shelly 

 matter ; a strong external ligament, but for which the hinge-line is not bent or 

 thickened. 



This differs from the typical Asthenothcerus in its porcellanous, instead of 

 earthy, shell-substance, destitute of granulations; its completely closed valves; 

 in possessing an external ligament ; and in the filling of the apices of the beaks 

 with a solid shelly mass on which the extraordinarily large and strong arched 

 ossiculum is planted, as on two pedestals. 



It is dedicated to Miss Katharine J. Bush, of New Haven, whose excellent 

 work on Mollusca I have had frequent occasion to refer to. 



