26 MEMOIRS OF THE STATE MUSEUM 



or that along the hinge, being thicker and stronger than the remainder, is 

 more often preserved, and the series is only complete in the representation 

 of this portion, although there are several small specimens which are sufifi- 

 ciently entire to show the early form of the shell. 



As in the other species which, in their mature proportions, depart from 

 the type of structure in the group, the incipient shell is found to revert to 

 the primitive form. The full-grown examples of this species are concavo- 

 convex, the concave valve being the ventral, while in the young, the ventral 

 valve is the more convex. This change in the relative convexity of the 

 valves does not begin until the individuals are about half-grown, and is pro- 

 duced by the gradual deflection of the margin with the increase in the size 

 of the shell. 



The development of the features of the hinge is very characteristic, 

 and, as in the other strophomenoid forms, is of primary interest. Both the 

 dorsal callosity and pedicle-tube continue to increase in size with the 

 growth of the shell, from the incipient form to maturity. 



SPECIFIC CHARACTERS 



Mature Form (plate 3, figs. 2, 2«, 8). Shell semi-elliptical, wider 



than long, the greatest length being along the hinge. The body cavity is 

 very shallow, and the shell has a concavo-convex form. 



Dorsal valve flat in the upper part, moderately convex in front. Ven- 

 tral valve slightly convex on the umbo, and concave over the remainder of 

 the valve. Hinge-area formed by both valves. Ventral area the wider, 

 carrying in the center a small conical pedicle-sheath which is usually min- 

 utely perforate at the apex. Dorsal area linear, with a callosity in the 

 middle, under the pedicle-tube of the opposite valve. 



Test thin, surface ornamented by about fifty alternating radii, with 

 three or four fine filiform strise in each interspace ; also crossed by fine 

 irregular striae of growth. 



