662 F. C. GREENE 



with a relatively short hinge-line and the shell longer and narrower 

 than the adult of C. granulifer from Grand Summit, and it is found 

 that all these come from a much lower horizon. Thus it will be seen 

 that they correspond to the early ephibic stage of our series. This 

 throws them into the species C. granulifer as already done by Schu- 

 chert and Weller. 



Ger otitic stage. — In the case of the gerontic form we find the same 

 thing to be true. Meek and Hayden' finding a form with mucronate 

 cardinal angles, longer hinge-line, much thicker valves, and, as they 

 say, " the area of its smaller valve ranges more nearly at right angles to 

 the plane of the shell, than in the IlHnois species"^ (C. smithii), they 

 called it a new species, although they suspected it of being allied to, 

 or possibly the same species as, C. smithii. The above characters are 

 found in the gerontic stage of the Grand Summit specimens, and 

 Meek and Hayden's material was from a higher horizon than the 

 Florena shales. The fact that the area of its smaller valve ranges 

 more nearly at right angles to the plane of the shell than in the Illinois 

 species, is due directly to the fact that it is a gerontic form. One of 

 the characteristics of this stage is the thickening of the shell, especially 

 in the pedicle valve, material being added to the inner surface of the 

 shell. To compensate for this, and keep the visceral cavity its normal 

 size, shell material is added at the hinge-line, which thickens the 

 cardinal area of the pedicle valve and forces the pedicle valve back, 

 making the hinge-line of this valve more nearly at right angles to that 

 of the brachial valve (Plate II, Fig. 12-13). 



In Prosser's collection from the Neosho (the next horizon above 

 the Florena) near Strong, Kansas, the majority of the specimens 

 exhibit a form remarkably like the gerontic stage in the Grand Summit 

 series. 



A collection from the Oread limestone at University Hill, Law- 

 rence, Kans., made by Mr. Chas. D. Ise, bears out the foregoing 

 conclusions to a remarkable degree. The figures (Plate III, Figs. 

 14-17) show a form identical in shape with the early ephibic stage in 

 the series. The hinge-line is short and the cardinal angle is nearly or 

 quite 90°. There is a tendency in many of the specimens to develop 



I Meek and Hayden, Pal. Up. Mo., ■ , PI. I, Fig. 5 a~e 

 ' Tbid. 



I 



