- 206 - 



Reraarlrs «- -^'he small -valves of this stron^-lj- sculptured form 

 are numerous. Ouppy proTDa-Qly did not .include the fir'"+^ ---' teriir and 

 the last posterior rihs in his enumeration, for none of the valves 

 at hand have less than nineteen ribs and the common nujnher is 

 twenty or twenty-one. Occasionally a much smc'' " ■ '"^' rihs is inter- 

 calated in one of the interspaces near the posterior margin. 

 Young valves are more orbicular than adults, the terrace on the 

 mediaji ribs is obsolete and the nodules on the ventral portions 

 of the "oosterior ribs display a tendency to be m.ore scabrous. A- 

 mong^ adult shells the characters are decidedly constant, except 

 for a slight shift in either direction ?ith regard to the number 

 of ribs. 



This S'oecies is characterized by its small size, subquad- 

 rate outline, with stroi^ sculpture of the V. a"! ticoste.t-'^ ^-I'VQ, 

 the terraces being prominent on the median "portion of the shell 

 and the norlulations strong. Dall has described a Ohi^' Prm., 



Y. hadra (l), '"hich is larger, less quadrate, with more prominent 

 4 — — — — — — — — — — — — — — ___^__ __ _______ ____ 



(1). loc. Git., -op. 1429-1450, pi. 53, figs. i:j., IS, 190?. 



umbones, wider and less strongly nodula tei? ribs and the terraces, 



if present, wealc. The Tampa silex and Oak Grove V. him.erta Dall (2) 



(21. Id m, p. 1430, pi. 53, fi-. 1?. 



