36 MEMOIRS OF THE STATE MUSEUM 



valve eight. No concentric growth-lines are apparent. Average dimen- 

 sions 8x6 mm. 



Variations from the normal adult. Two plications sometimes occur 



in the sinus, and in such cases they are each stronger than the single sinal 

 plication in the normal adult. The addition of the plication to the sinus 

 increases the number in the fold to three, and the total number of plica- 

 tions on the shell by two. 



Initial shell (Compare plate 4, figs. 9, ()a, gd.) Two individuals, 



one measuring 1.2x8 mm., the other 1.4x9 mm., apparently indicate the ini- 

 tial stages in the growth of this shell. Neither of these examples has 

 served well for illustration, on account of the lack of well-defined details, 

 but they may be described as follows : — Attenuate, subspatulate. Ventral 

 valve with erect, straight beak ; cardinal area high, convex, with a promi- 

 nent dorsum. Dorsal valve flattened or slightly sinuate. Features of 

 the cardinal area not discernible ; from analogy, the foramen would be 

 triangular and unobstructed. In figures 9-9^5, which show a secondary stage 

 of growth in the shell, the portion included within the first growth-line will 

 represent very well the characters of the primitive shell. 



* GENERAL DEVELOPMENTAL CHARACTERS 



The gradual incurvature of the beak and consequent concealment of 

 the ventral foramen may be assumed from the foregoing. It harmonizes 

 with the associated species of the same genus in the slight variation in the 

 form and proportions of the foramen in consecutive stages of growth, as 

 well as in the reversal of the embryonic fold and sinus to the mature sinus 

 and fold. The plications of the latera seem to appear simultaneously after 

 the first varix, as shown in the, figures referred to, and their number does 

 not change materially until maturity. The embyronic sulcus on the dorsal 

 valve, correlate with the ventral dorsum in the primitive stage, is continued 

 at maturity into the median sulcus separating the two plications of the 

 dorsal fold. 



