40 MEMOIRS OF THE STATE MUSEUM 



SPECIFIC CHARACTERS 



Mature Form (plate 4, figs. 2, ia, zb). Shell transversely sub- 



elliptical ; length and width about equal. 



Ventral valve shallow ; beak high, acute, somewhat attenuate, with the 

 apex slightly incurved, but not concealing the triangular unclosed foramen 

 which reaches entirely across the cardinal area. At its apex the foramen 

 encroaches slightly upon the umbo, and is narrowed somewhat toward the 

 base by the imperfectly developed deltidial plates. A median depression 

 makes its appearance at about one-third the distance from the umbo to the 

 anterior margin, and soon develops into a deep sinus with sharply sloping 

 sides. 



Dorsal valve deeper and more gibbous ; beak inconspicuous, and 

 incurved beneath the ventral foramen. A strong median fold corre- 

 sponds in development with the median sinus of the opposite valve. 



Surface marked by strong, simple, subangular plications, invariably 

 two upon the fold and one in the sinus, with six on each of the latera, 

 making thirteen on the ventral, and fourteen on the dorsal valve. Of these, 

 the plications near the cardinal margin are low and incipient, but the full 

 number becomes permanent early in the history of the individual. Faint 

 concentric growth-lines are sometimes visible. Dimensions of average 

 adult i ix 1 1 mm. 



Abnormalities at maturity. The variations from the normal ma- 



ture form are, as far as observed, wholly due to continued internal growth 

 after individual maturity has been attained, and this is to be regarded as 

 the concomitant evidence of senescence. There may be either a marginal 

 thickening, which gives the shell a truncate appearance, or a general internal 

 thickening, making the shell unusually gibbous, and forcing the ventral beak 

 over upon the dorsal umbo. 



Incipient Form (plate 4, figs, i, 10). The youngest individual 



observed measures 2.75x2 mm. ; outline subovate, valves regularly rounded, 



