44 MEMOIRS OF THE STATE MUSEUM 



C. Forms with four plications in the ventral sinus. This variation is met 

 with very infrequently, but two individuals having been obtained. While 

 agreeing in size with the normal adult, the crowding of the sinus with pli- 

 cations tends to obliterate both it and the fold upon the opposite valve. 

 Both individuals show the interesting fact that upon the dorsal valve where 

 the fold bears five plications, i. e., four sulci, the embryonal sinus is contin- 

 uous with the third of these sulci, in one instance numbering from the right, 

 in the other, numbering from the left. Of the five plications which are thus 

 separated into groups of three and two, it is noticeable that the outer 

 member of the group of three is both less elevated and shorter than any 

 other upon the fold. 



Monstrous Forms. The sole evidence of monstrous growth that has 

 been observed is an asymmetrical development of the plications upon the 

 dorsal fold. Examples bearing three plications upon the fold, in rare 

 instances have one of the plications very large and two quite small, making 

 one broad and one narrow sulcus upon the fold. . The phenomenon may be 

 due to the strongly developed tendency of the embryonal sinus to maintain 

 its continuity with a median sulcus even at the expense of the symmetry 

 of the shell. 



Initial Shell (plate 3, figs. 17, i 7#). The initial shell in our series 



of Rhynchonclla indiancnsis measures .65 mm. in length by .54 mm. in width. 

 It is broadly ovate or subpyriform in aspect, convex posteriorly, and 

 depressed toward the anterior margin. Ventral valve with the umbo 

 prominent, the beak elevated and erect, with the apex rounded ; cardinal 

 margins rapidly sloping. Foramen subtriangular, apical portion broader 

 than usual in the incipient stages of plicate shells ; margins not thickened ; 

 deltidial plates absent. Dorsal valve with a rounded, inconspicuous beak. 

 Surface of both valves quite smooth. A median depression is noticeable 

 on the dorsal valve near the anterior margin, making this margin sinuate. 

 This embryo is the smallest that has been found for any of the series 



