56 MEMOIRS OF THE STATE MUSEUM 



SPECIFIC CHARACTERS 



Mature Form (plate 5, figs. 2, 2a). Shell ovate, generally longer 



than wide, both valves almost evenly convex, and of about the same depth. 

 Anterior margin generally slightly emarginate, on account of the median 

 sinus which exists on both valves. In rare instances, a low median fold is 

 developed near the margin of the dorsal valve. 



Veniral valve with the beak much elevated above the dorsal, and in- 

 curved, so that the plane of the foramen is parallel to the axial plane of the 

 shell. Foramen circular, or slightly subtriangular ; deltidium generally 

 obscure on account of the infolding of the beak, but showing the triangu- 

 lar deltidial plates. 



Dorsal valve regularly arcuate, except at the posterior extremity, where 

 the beak is closely incurved beneath the ventral umbo. The median sinus 

 usually carries from three to five plications, but sometimes becomes filled up, 

 in advanced growth, by the crowding of these plications. Ventral valve 

 with a well-marked sinus, generally bearing three plications. The sinal pli- 

 cations on both valves take their origin in front of the beak, and are of 

 interstitial growth, a fact which does not hold true for any of the other 

 plications. The surface is marked by from eight to twelve rounded, con- 

 tinuous plications on each side the sinus of either valve, all of these ex- 

 tending to the beak, with the possible exception of the more obscure ones 

 on the cardinal slopes. Only in rare instances and abnormally, do these 

 plications increase by interstitial addition. Imbricating lines of growth are 

 often present, and fine concentric striae are sometimes discernible. 



The mature individuals of Retzia evax divide themselves into three 

 groups, based on their relative proportions : 



{a) Normal form, in which the length and width are equal. 



(^) Long form, in which the length is greater than the width. 



(r) Broad form, in which the length is less than the width. 



In frequency of occurrence, the form {b) almost equals the normal, 



