DESCRIPTIONS OF SPECIES. 358 



number is six or seven. Even in specimens of the same size and apparent^ the 

 same age, differences occurred in the number and arrangement of the spines developed, 

 a-t least in the number which could be ascertained. There was also great variation 

 in the plicated or sinuate condition of the shell. The dominant type had a strong 

 sinus of about the degree of development shown in Norwood and Pratten's figures of 

 Cli. flemingi, but variation extended on the one hand to an incipient obsolescence of 

 plication and on the other to a condition as strongly folded, in some instances, as 

 Ch. vemeuilianus, where figured in the original description. Another point of varia- 

 tion which may be mentioned here is in the proportions of the shell, which ranges 

 from distinctly transverse to sutaquadrate, and in the prolongation of the cardinal 

 extremities. A transverse shape, produced cardinal line, and a deep fold and sinus 

 usually go together in this species, so far as my observations extend. 



The number of striaj developed depends, of course, in large measui-e upon the 

 size of the shell, but in the size of the striae and the number occurring in a given 

 space no considerable variation was observed. Thej' do vary appreciablj', however, 

 in the strength with which they are developed, in some cases being raised and strong 

 and in others obsolescent. This difference can not be ascribed to erosion, as in 

 many cases the delicate concentric growth lines could be detected. These are unlike 

 the even, threadlike, concentric strife covering the coarser radiating ornamentation 

 on many Lower Carboniferous members of the genus, but are rather irregular and 

 minutely lamellose. 



The pittings between the striae mentioned by the authors in connection with Gh. 

 fjemi/nigi could be seen in most of the specimens examined, though, of course, 

 preservation j)lays an important part in concealing and revealing such minute 

 characters. 



Much the greater part of the material examined, therefore, seems to belong 

 rather to Cli. flemnvngl than to Ch. verneuilianus. In several instances a few 

 individuals, especially by reason of increased gibbosity and an exaggerated develo^D- 

 ment of fold and sinus, approach the limits occupied by the species Ch. verneuiliamus. 

 Whether the}^ are true representatives of that species is still a matter of doubt in ni}' 

 mind, but if they can be correctly so identified, I think one name might well be 

 reduced to varietal import, as every gradation seems to occur between the two types. 

 In view of these observations Ch. flemingi would more properly be retained as the 

 specific appellation, not only because it has priority among the species described by 

 Norwood and Pratten, but because it seems to be the abundant and dominant type of 

 which the other is but a variant. < 



The small varietj^ from Caseyville mentioned above is transverse, with well 

 developed sinus, hinge line much longer than the width of the shell below, and with 

 slightly obsolescent strise. 



14364— No. 16—03 23 



