62 MEMOIRS OF THE STATE MUSEUM 



ticed in either of the species Rhynchonclla xohitii and R. neglecta. Our 

 material represents all developmental stages between the limits of these 

 dimensions; 2x1.6 mm. (incipient shell) and 7x6.5 mm. (maturity). In 

 its youngest stages, it shows a certain degree of similarity with Retzta 

 evax, especially in the sinus on both valves, and in the sinal plications. 

 The greater number of the latter in R. evax, as well as the more numerous 

 lateral plications, will serve to obviate confusion here. 



SPECIFIC CHARACTERS 



Mature Form (plate 5, figs. 11, \ia, \ib). Shell small, rotund, in 



outline broadly ovate to subpentagonal. Valves of equal convexity. 



Ventral valve with umbo prominent, attenuate, erect, and slightly in- 

 curved at the apex ; cardinal margins not excavate, sloping with a faint 

 curve to the sides, whence they round to the anterior edge, which, in the 

 sinal region, is nearly straight ; cardinal area distinct ; foramen circular ; 

 deltidial plates prominent. 



Dorsal valve subcircular in outline, arched in the umbonal region ; 

 beak well-defined, apex concealed. In the umbonal region, the median 

 portions of both valves are slightly more convex than elsewhere, but this 

 prominence disappears toward the margins, the valves becoming slightly 

 flattened and depressed on the median region near the anterior margin, 

 making a low sinus on the ventral, and a low, depressed fold on the 

 dorsal valve. Both fold and sinus may bear one, two, or three small, often 

 faint and unsymmetrically developed plications, the strongest of which 

 may have its origin in the umbonal region, while the others rarely extend 

 more than half-way across the shell. On each of the latera are four or 

 five strong, angular, simple plications, making thus from nine to thirteen 

 plications on each valve. The increase in these takes place altogether on 

 the fold and sinus, the full quota of lateral plications appearing early in the 

 history of the individual. The plications are covered by numerous fine, 

 concentric growth-lines more noticeably developed near the margin, and at 

 intervals becoming varicose. 



