REPORT ON THE CRINOIDEA. 277 



by different palaeontologists, e.g., on pi. 53 of Buckland's Geology and Mineralogy, 

 figs. 9-13 ; on Tab. 101 of Quenstedt's Encriniden, especially figs. 24, 27, 33, and 37; 

 and also on pi. 12 of the Messrs. Aiistin's Monograph, figs, d,f, k, n, o, r. None of these 

 authors, however, seem to have noticed the distinction of this type of joint from that of 

 the ordinary Pentacrinidse, either recent or fossil ; and attention was first drawn to it by 

 de Loriol, as will be pointed out subsequently. 



In the ordinary PentacrinidEe, as in the multiradiate Comatulse, there is no special 

 regularity in the mode of division of the secondary and tertiary arms. The twenty 

 secondary arms borne upon the distichal axillaries may either remain simple, or fork 

 again, once or oftener. But in most cases the forking is very irregular. Secondary 

 (palmar) axillaries may appear upon any of the four secondary arms; and the two 

 tertiary arms borne by them are of equal size, and have equal power of forking again, 

 though as a rule they do not all do so. A good instance of this is shown in Miller's 

 figure ^ of Pentacrinus asterius {caput- Medusce) , which is represented more diagrammati- 

 cally in Quenstedt's Tab. 97, fig. 3. A similar arrangement has been described by 

 Liitken in Pentacrinus miilleri,^ while it also occurs in Pentacrinus maclearcmits, 

 Pentacrinus ivyville-tliomsoni, and Pentacrinus alternicirrus (PI. XIV. ; PI. XV. fig. 1 ; 

 PI. XIX. fig. 2 ; PI. XXV. ; PI. XXVI. fig. 4). In all these cases the secondary (and 

 tertiary) axillaries are limited to the outer arms of each successive pair, so that the arrange- 

 ment of the arms on the ray is 2, 1 ; 1, 2 ; or 2, 1, 1 ; 1, 1, 2.^ But the two (or four) 

 inner arms are equivalent to the outer ones in all respects, neither of them dividing again. 



"While the arm-division in Extracrinus proceeds to a much greater extent than in 

 Pentacri7ius, it is confined as a rule to the sides of the ray, only the outer arm of each 

 successive pair bearing axillaries, just as in the secondary and tertiary arms of 

 Pentacrinus asterius, Pentacrinus miilJeri, &e. The four tertiary arms which spring 

 from each pair of palmar axiUaries are rarely of equal size, and never absolutely equi- 

 valent. The two inner ones are usually rather the smaller, and except in some forms of 

 Extracrinus suhangidaris do not divide again. Each of the larger outer arms, however, 

 divides again after a few joints, but the division is unequal. The smaller inner face of 

 the axillary, i.e., that turned towards the other axillary, bears a slender armlet; while the 

 main arm-trunk is continued directly onwards without change of direction. It gradually 

 diminishes in size, and gives off" at short intervals a series of slender armlets from its 

 inner side, but it never really forks. But for the pinnules borne by it and its 

 subordinate armlets, one would be almost inclined to say that the distichal axillary bears 

 two secondary arms which have long slender pinnules placed at intervals upon their 

 adjacent inner faces, but none whatever upon their outer sides. These organs are real 



1 Op. cit., p. 48, pi. i. ^ Om Vestindiens Pentacriner, loc. cit, pp. 203, 204. 



' The Caribbean Antedon spinifera often shows exactly the reverse condition to this. Palmar axillaries are frequently 

 only developed on the inner pair of the four secondary arms, so that the grouping on each ray is 1, 2 ; 2, 1. 



