MORPHOLOGY 47 



frequently the smallest; and in one singular species, Cyathocrinus maxvillensis 

 Whitfield, the two posterior rays are dwarfed. In the Camerata also the lateral 

 rays are occasionally of inferior size. But in none of those groups is there a 

 case where the disproportion is so great as in Cholocrinus. 



Throughout the group there was a strong tendency in the arms to coil 

 tightly upon themselves over the tegmen (PL XXXV, fig. 16). It is very rare 

 to find a specimen with the arms spread out so as to expose the ventral side as) 

 often occurs in other groups, except in one species of Onychocrinus; even in 

 this, however, it is only the lower parts of the rays that are exposed, the 

 ramules themselves being always tightly folded. Owing to this infolding the 

 arms give an impression of stoutness which is not true in all cases for their 

 distal portions ; these are sometimes exceedingly slender. 



As a general rule in the crinoids enlargement of the arms is accompanied 

 by reduction in size of the calyx, and vice versa. Thus the ponderous arms of 

 Arachnocrinus 1 belong to a calyx which is little more than a center of connec- 

 tion between them, while Hadrocrinus plenissimus,' 2 with almost the largest 

 calyx of any crinoid, is now known to have a great number of short, thread- 

 like arms. Among the Flexibilia the Lecanocrinidae, and to some extent the 

 Sagenocrinidae — e. g., the genus Sagenocrinus — have relatively a large calyx 

 and small arms, while the reverse is the case with many of the Taxocrinidae. 



Pinnules are entirely wanting in the Flexibilia. Their absence is one of 

 the characteristics of the group. There are ramules in the heterotomous types 

 which may be considered an intermediate stage between pinnules and arms; 

 and in one of the later species of Onychocrinus, with ramules branching alter- 

 nately from every second brachial, there is the nearest approach to pinnulation 

 known in the group; but the resemblance is only slight. That ramules are 

 somewhat different elements from pinnules is indicated by the fact that in some 

 Melocrinidae, and in Lampterocrinus, the ramules themselves are regularly 

 pinnuliferous, and they frequently branch on the same principle as arms. 



Clark has shown 3 that the free arms of the crinoids are composed of an 

 extension of the boundary between the primarily skeleton-forming dorsal sur- 

 face and the perisomic ventral surface, so that any series of ossicles on the 

 border between these two surfaces, and composed in equal parts of each, will 

 assume the structure common to all the processes arising in the same region, 

 and will take on from the beginning the structure of the crinoid arm. Thus it 

 is possible to have an interradial arm, such as is found in the posterior inter- 

 radius of the so-called Thaumatocrinus renovatns of P. H. Carpenter, arising 



1 Springer, New Am. Foss. Crinoidea, Mem. Mus. Comp. Zool., Vol. XXV, no. 3, 191 1, PI. I, fig. 1. 

 " Wachsmuth and Springer, N. A. Crin. Cam., Mem. Mus. Comp. Zool., Harvard, Vols. XX, XXI, 

 1897, PI. XXIV, fig. 2a. 



3 Journal Washington Acad. Sci., vol. 2, 1912, p. 311. 



