. Annual report of the regents of the university of the state of New York on the condition of the State Cabinet of Natural History and the historical and antiquarian collection annexed thereto. y satisfactorily determined, while in others theexistence of the area has been ascertained. In older specimens the shell is oftenmuch thickened, and the cavity partially filled by the substance of the valve. It has not been possible, with the specimens in my possession, to determinewhether this feature is characteristic in all stages of growth; but since it occursin some half-grown specimens which are b
. Annual report of the regents of the university of the state of New York on the condition of the State Cabinet of Natural History and the historical and antiquarian collection annexed thereto. y satisfactorily determined, while in others theexistence of the area has been ascertained. In older specimens the shell is oftenmuch thickened, and the cavity partially filled by the substance of the valve. It has not been possible, with the specimens in my possession, to determinewhether this feature is characteristic in all stages of growth; but since it occursin some half-grown specimens which are but moderately gibbous, and where thebeak is but moderately incurved, I infer that this character is assumed at an earlyperiod 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 ventricoseforms of the Lower Helderberg group, I have been unable to detect any featureof this kind. The accompanying figures are of the dorsal and ventral valve of a specimen ofthis species, where the shell is much thickened. Fig. 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 usedthe letter d to designate the area, its use being to indicate in shells of this kind the deltidialplates. Fig. 14 is a figure of a smaller individual, enlarged, where the beak is but littleincurved, the foramen well defined, and the solid area well preserved, showinglongitudinal striae. It may be suggested that the cardinal area results fi-om the bending inwards andcoalescing of the deltidial plates, but its appearance scarcely corresponds withsuch a change in these parts. The area is distinctly defined, making an angle withthe outer shell at its lateral margins, which limits it as strongly as the cardinalarea in Spirifer, while the course of
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectscience, bookyear1853
