50 MEMOIRS OF THE STATE MUSEUM 



The ventral valve is uniformly convex in all incipient specimens. The 

 sinus develops at the same period, and in conformity with the fold of the 

 opposite valve. The beak of the initial shell is broadly triangular, perfo- 

 rate at the apex, and directed outwards. It gradually becomes narrower and 

 less oblique with advancing growth, and lies in the axis of the shell in full- 

 grown specimens. The initial perforation is a small truncation of the beak, 

 confluent with the open area below. {Sec figure i6, plate 4.) 



Surface ornaments. The prevailing number of plications is eight, al- 

 though it varies from seven to ten in some specimens. The entire number 

 appears at an early period of growth, and in this respect, the species offers 

 a marked difference from some of the forms of Rhynchonella already con- 

 sidered, in which the plications increase by pairs. In a specimen 1.5 mm. 

 long, they first appear at about one-fifth the length of the shell from the 

 beak. Four plications are included in the depression of the dorsal valve in 

 the incipient stages, and the two central ones finally become elevated, 

 forming the fold in the full-grown shell. Upon approaching maturity, three 

 of the plications in the ventral valve are depressed, the middle one ulti- 

 mately much more than the others, forming the single strong plication at 

 the bottom of the sinus. 



No concentric striae are shown on the initial shell of our series. In a 

 specimen 3 mm. in length, these begin to develop over the outer third of 

 the surface, as shown in figure 13, plate 4. 



Cardinal area. The foramen is at first a broad triangular opening, 

 wider than high, with sharp margins, and truncating the beak of the ventral 

 valve. The lateral margins are thickened in a specimen 3 mm. in length 

 (plate 4, fig. 13), and thejieight and width of the area are equal. These pro- 

 portions of height and width are preserved to maturity, although in some 

 specimens the area is higher than wide. No deltidial plates have as yet 

 appeared, but in the next stage, including individuals having a length of 

 4.5 mm., there are two narrow deltidial plates developed from the sides of 



