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REPORT ON THE CRINOIDEA. 49 
first axillary (inclusive) is particularly advisable in the case of the Neocrinoids. For 
among all the members of this sub-class which have ten or more arms,’ Metacrinus is the 
only genus besides Plicatoerinus in which the second joint beyond the primary radials is 
not an axillary, a character which has-elsewhere been pointed out as distinguishing the 
Neocrinoids from the Palzocrinoids.? In the various types of Comatulze and in some 
species of Pentacrinus there is a similar constancy in the number of joints which intervene 
between the successive axillaries of the dividing arms. I have therefore found it 
convenient ® to give special names to. the joints composing the primary and secondary 
arms respectively, and to restrict the term “brachials” to the joints composing those 
portions of the arms which undergo no further division. 
The joints of the primary arms may be called “ distichals,” a term no longer used with 
the precise meaning which Miiller attributed to it;* while the joints of the secondary 
arms (if there be any) may be termed “palmars.” These names are of much use in 
descriptions of Comatulz ; for in this family the number and character of the segments 
between the successive divisions of the arms exhibit variations which are, to a great 
extent, constant in different species, and thus give us the means of classifying them into 
larger or smaller groups.” 
The Pentacrinidze, however, exhibit a much greater irregularity in this respect ; and 
they also present more exceptions to the following rule, which holds good in almost all 
the Neocrinoids. The first two joints beyond every axillary of the dividing rays are 
united to one another in the same manner, either syzygy or bifascial articulation, as the 
second and third radials are. Thus, for example, there is a syzygy between the two outer 
radials of Encrinus, and another between the two lowest brachials. In Apioerinus and 
Millericrinus the corresponding joints are respectively united by bifascial articulations. 
This rule holds good in all the ten-armed Comatulx, whether the joints are articulated 
(Antedon rosacea) or united by syzygies (Actinometra solaris); and it is equally true in 
all the many-armed species with the exception of two groups of Actinometre, together with 
a few unusually aberrant types. In one group, which is represented by Actinometra 
multiradiata, the two outer radials and also the first two of the three distichals are 
articulated by ligaments only; but in all the subsequent arm divisions there is a 
muscular joint between the first two segments after each axillary, and the second one is 
traversed by a syzygy, whether it be itself an axillary or a free brachial, while the first 
bears a pinnule. Another variation occurs in Actinometra typica, Actinometra nove- 
guinee, and their allies. These forms have three distichals in the primary arms, the 
first two of which are articulated, while the axillary is a syzygy; but the two outer 
1 This passage does not refer to Promachocrinus, in which genus there are ten primary rays. 
2 On Allagecrinus, &e., Ann. and Mag. Nat. Hist., ser. 5, vol. vii., 1881, p. 296. 
3 Actinometra, Trans. Linn. Soc. Lond., (Zool.), ser. 2, vol. ii. p. 24. 
+ Bau des Pentacrinus, p. 31. 
5 Classification of Comatule, Proc. Zool. Soc. Lond., December 1882, pp. 746, 747. 
(ZOOL. CHALL, EXP.—PaRT xxx11.—1884.) 7 
