136 PALJEONTOLOGY OP NEW-YORK. 



form should have been regarded as a distinct species. The casts show the impros- 

 sion of a strong cardinal process and widely diverging dental lamella;, with 

 deeply pitted surfaces towards the exterior margin. Some of these forms are illu- 

 strated on figures 13 a- d on Plate 21 ; and some of the larger individuals on 

 Plate 22. 



In these specimens, the cardinal extremities are often extended beyond the 

 width of the shell below, as shown in figure 13 6, c; but the greater number 

 preserve a form similar to that given by Mr. Conrad. 



I have before me at this time, through the kindness of Dr. Norwood, the origi- 

 nal specimens from which C. martini, C. maclurea and C tuomeyi were described, 

 as well as others corresponding to C. littoni; together with a considerable col- 

 lection made by myself at the same locality from which they were obtained. 

 After careful comparisons of all those from the western locality,* I am unable to 

 recognize any specific difierence among them ; and they correspond as closely 

 with C. coronata of New-York, presenting the same varieties of form, hinge-exten- 

 sion, convexity, and flattening or depression of the middle of the ventral valve, 

 and the same variety in the bifurcation and intercalation of striae. I feel compel- 

 led, therefore, to regard them all as varieties of the eastern type. 



The C. konincki of Norwood and Pratten, I am disposed to believe only a 

 variety of the same form. A specimen from the original locality, near Jonesboro, 

 Illinois, labelled by Dr. Norwood, presents no reliable characters for separation 

 from some of the specimens from the Bake-oven. 



The original of C. tuomeyi, figure 9 a, b, of Plate xxi, may be compared with 

 the young of C. coronata, figures 11 a,b, of the same plate; and C. maclurea, figure 

 11, maybe compared with C. coronata, figure 10 A, and others of the same; 

 while C. martini, figure 12, may be compared with C. coronata in figure 10 e, or 

 with figure 13 c. The C. littoni is intermediate in size between C. tuomeyi and C. 

 maclurea, and may be compared with C. coronata, figure 10 g. 



The general aspect of all the western specimens, which are in limestone, is 

 that of having coarser and stronger strise than the C. coronata of the soft shales 

 of the Hamilton group in New- York. A careful measurement of the stria) on diffe. 

 rent parts of the shell, in a number of specimens from the central and western 

 counties of this State, shows that they range from 9 to 15 strios in the space of 

 two-tenths of an inch ; the majority being from twelve upwards in the centre, 

 and from nine upwards near the front and basal margin of the shell. As a rule, 

 the smaller specimens have the finer strioe. 



Specimens of C. tuomeyi give from 9 to 12 striaD in the space of two-tenths of 

 an inch : C. maclurea, which is larger than C. tuomeyi, gives 10 and 11 striaa in 



* The Bake-ovcD, Illinois, on the Mississippi river. 



