224 PROCEEDINGS OF THE ACADEMY OP [1881. 



In this lot of crinoids there are two more specimens, one having 



112 111 



twenty -one arms distribnted thus : 1111 1122 , and the other twenty- 



221,222 22 



seven, thus: 22121112. Both agree with the two preceding 



species except in the arm formula, but even this is fundamentallj^ 

 identical with E. originarius, the simple arms of the latter being 

 in part replaced by pairs. None of our Bono specimens have the 

 doubh; arm structure throughout all the rays, but we obtained 

 from Edwardsville and Canton several specimens in which that 

 feature prevails, and for which we propose the name Eretmocrinun 

 adultus. The two irregular Bono specimens may not be the 

 intermediate forms between that species and JiJ. originarius^ but 

 this is probable, and they show how the double arm structure 

 became introduced. 



Scarcely less interesting are some specimens of Batocrinus. 

 Among sixteen examples of a form which we call B. Whitei — eleven 

 from Bono, two from Canton, and three from Edwardsville — there 

 are fourteen with the arm formula: II, while two of them have 

 four arms to the posterior ray. No double arm structure has been 

 observed in this species at Bono, but its apparent representative 

 at Crawford sville has always two arms from each opening. B. 

 Indianensis has the same form and ornamentation as B. Wliilei, 

 and the same arm formula — with substitution of double for single 

 arms — and it evidently bears the same relation to that species as 

 B. ChrysH to B. Lovei, and E. adultus to E. originariui-. 



It would be interesting to pursue this line of examination further, 

 and trace the relations subsisting among other groups similarly 

 connected. But we have perhaps gone far enough to serve our 

 present purpose. It is to be observed, however, that the import- 

 ance of this kind of investigation, in its bearing upon sj- stematic 

 classification, can scarcely be overestimated. It has to do with 

 the principles which lie at the very bottom, and it is only by the 

 study of these relations, of the exact anatomical changes which 

 produced individual variation, and in time permanent modification 

 of forms, that we can hope to arrive at a correct understanding of 

 the groups in nature, or be able to make scientific discrimination 

 of families, genera, species and varieties. 



Many species have been made, upon mere differences of growth; 

 some upon unimportant variations in the arm formula; some upon 

 abnormal development in certain parts of the bod}' ; others upon 



