68 MEMOIRS OF THE STATE MUSEUM 



with individuals having a length of 2.5 mm. and a width of 1.75 mm., the 

 adult form measuring ,9 mm. in length by 7.5 in width. 



In all stages of growth earlier than that approximately indicated by a 

 size of 6x4 mm., it is very difficult, and from our observations impossible, 

 to draw the line of separation between this species and M. nitida, and the 

 fact which has been demonstrated for Or this elcgantula and O. hybrida, viz.; 

 that in the earliest growth-stages no specific differences are manifest, will 

 probably be found to hold good for these two species also.* And in the 

 latter case a considerably larger size is attained by the embryo forms than 

 is reached by the former species, before the differential characters are 

 assumed. This is due to the fact, that these two species, when mature, 

 have essentially no surface sculpture, and differ less in general form and 

 outline than do the mature individuals of Orthis. 



SPECIFIC CHARACTERS 



Mature Form (plate 7, figs. 12, \2a, \2b). Shell subpentagonal or 



ovate ; beak erect, acute, and prominent, rapidly widening toward the base. 

 Lateral margins nearly straight for about one-third the length of the shell, 

 thence rounding to the anterior margin. Valves about equally convex, 

 giving the shell a sublenticular contour. 



Ventral valve with attenuate, straight, or slightly arcuate beak. Fora- 

 men triangular and without deltidial plates. 



Dorsal valve more nearly subpentagonal in outline ; beak incurved into 

 the foramen of the ventral valve. 



Surface smooth, or in rare instances showing a faint pseudo-punctate 

 appearance which is entirely superficial. Dimensions of average adult 

 9x7.5 mm. 



*In accordance with the intention already expressed, not to enter into discussions of 

 synomymy and generic values, we have followed Mr. Hall in referring these species (M. recti- 

 rostra and M. nitida) to different genera. We believe, however, that the character of the loop 

 will prove them to be congeneric. 



