68 



[Senate 



from the interior of the shell, there is a distinct foramen which passes out at the 

 apex or above the apex of the valve, a groove on the lower side always extending 

 thence to the apex. This area sometimes shows a longitudinal suture line, but this 

 feature is not always visible. In three well-preserved specimens, this area and the 

 foramen beneath have been very satisfactorily determined, while in others the 

 existence of the area has been ascertained. In older specimens the shell is often 

 much thickened, and the cavity partially filled by the substance of the valve. 



It has not been possible, with the specimens in my possession, to determine 

 whether this feature is characteristic in all stages of growth; but since it occurs 

 in some half-grown specimens which are but moderately gibbous, and where the 

 beak is but moderately incurved, I infer that this character is assumed at an early 

 period of growth. In the R. altilis of the Chazy limestone, there are some evi- 

 dences of the feature here described, but the specimens I have do not afford suffi- 

 cient proof At the same time, after an examination of the gibbous and ventricose 

 forms of the Lower Helderberg group, I have been unable to detect any feature 

 of this kind. 



The accompanying figures are of the dorsal and ventral valve of a specimen of 

 this species, where the shell is much thickened. 



Fig. 12 



Fig. 13. 



Fig. 12 & 13. Ventral and dorsal valves of the natural size. 



The letters refer to the same parts as in the figures 1 and 2 of a preceding page. I have used 

 the letter d to designate the area, its use being to indicate in shells of this kind the deltidial 

 plates. 



Fig. 14 is a figure of a smaller individual, enlarged, where the beak is but little 

 incurved, the foramen well defined, and the solid area well preserved, showing 

 longitudinal striae. 



It may be suggested that the cardinal area results fi-om the bending inwards and 

 coalescing of the deltidial plates, but its appearance scarcely corresponds with 

 such a change in these parts. The area is distinctly defined, making an angle with 

 the outer shell at its lateral margins, which limits it as strongly as the cardinal 

 area in Spirifer, while the course of the foramen beneath it is distinctly defined. 



Should the characters here described prove constant in this and other species, 

 it may be convenient to separate them under the subgeneric designation of Rhyn- 

 chotrema, in allusion to the apicial foramen.* 



* Since the preceding pages were prepared for the press, I have seen several valves of this 

 species from the shales of the Hudson-river group in Ohio. In one of four only -was there any area 

 preserved ; the specimens^ however, had been somewhat waterworn. 



