1885.] NATURAL SCIENCES OF tHILADELTIIIA. 345 



which are radial in position as radials,and all intermediate plates 

 as interradials, the species possesses 12 and more radials to the 1 ay, 

 and 100 and more plates in each interradial space — an enormous 

 increase over the plates in Acrocr. Wortheni. Such a wide differ- 

 ence in the number of interradial plates among species of the 

 same genus is certainly very remarkable, but might be accounted 

 for, as this class of plates is subject to great variation ; but a 

 numerical difference in the primary radials, if such w:is the case, 

 would be exceptional. The primary radials are elements which, 

 once developed, do not multiply, but their number is constant 

 throughout the genus, and we doubt if Acrocrinus forms such a 

 remarkable exception to the rule. It seems to us more probable 

 that only the two large, contiguous upper plates, and the small 

 triangular bifurcating piece succeeding them, are radials, that 

 only the three intervening pieces in a lateral direction are true 

 interradials, and that all lower plates, from the basals up, are 

 merely accessory pieces, which obtained their position, whether 

 radial or interradial, accidentally through their alternate arrange- 

 ment, and the regularity with which they are distributed. By 

 this interpretation the two species, which appeared to be so widely 

 distinct, are brought within the limits of the same rule — both 

 having the same number of radials, interradials and anal plates. 

 It is true that accessory pieces like these are not found dorsally in 

 any other genus of the Palseocrinoidea, but they are not uncom- 

 mon among Cystideans,and similar plates occur ventrall} 7 in some 

 of the larger Actinocrinidae and Rhodocrinidae, which, like those 

 of the calyx, increase numerically by age, being represented some- 

 times by a single ring, and again, in the same species, hy a wide 

 belt of pieces. The accessory pieces in Acrocrinus increased in 

 number by adding constantly new rings above the basals. This 

 is well shown by the small specimen of Acrocrinus Wortheni, in 

 which the plates of the latest ring are yet triangular, only the 

 upper portion being developed ; and it is further indicated by 

 the increase in the size of the plates, which is in an upward 

 direction. 



In two of the New York specimens, the arms are partly pre- 

 served, and in the third one also portions of the vault. Acrocri- 

 nus had a third primary radial, which had not been observed in 

 Acrocrinus Wortheni. It is triangular and resembles the small 

 second radials of Platycrinus, resting like those within the con- 



