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 Plicatocrinus 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 Palaeocrinoids.^ In the various tyj^es of Comatulce 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 Comatulaj ; 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 Pentacrinidse, 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 Aiyiocrimis and 

 MiUericrimis the corresponding joints are respectively united by bifascial articulations. 



This rule holds good in all the ten-armed Comatulse, 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 Actinometrse, 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 axiUary or a free brachial, while the first 

 bears a pinnule. Another variation occurs in Actinometra typica, Actinometra novcB- 

 guinece, 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 



> This passage does not refer to Promachocrinus, in wliich genus there are ten primary raxjs. 



2 On Allageorinus, &c., Ann. and Maij. Nnt. Hist., ser. 5, vol. vii., 1881, p. 296. 



3 Actinometra, Trans. Linn. Soc. Land., (ZooL), ser. 2, vol. ii. p. 24. 

 * Bail des Pentacrinus, p. 31. 



^ Classification of Comatiihe, Proc. Zoul. Soc. Land., December 1882, pp. 746, 747. 



(ZOOL. CilALL. EXP. — PART XXXII. — 1884.) Ii 7 



