ME. P. H. CAEPENTEE ON THE GENES ACTINOMETEA. 23 



In both these groups the primary arms do not subdivide ; so that the total number of 

 arms is limited to ten ; and we are as yet unacquainted with any Comatula in which the 

 second and third radials are united by a syzygy and there are more tban ten arms 1 . This 

 is, however, the case in Pentacrinus Millleri, in which, in like manner, the first and 

 second segments beyond every brachial axillary are also united by a syzygy 2 . On the other 

 hand, Pentacrinus asteria, L. (=P. caput-Medusce, Midler), is remarkable for having 

 muscles between the second and third radials as well as between the first and second 3 . 

 In the same manner the first and second segments beyond every axillary are united 

 by muscles, and the syzygium is between the second and third segments '. 



In nearly all the Comatulce with which we are acquainted, with the exception of Act. 

 Solaris and the species most nearly allied to it, the second and third radials are united 

 by ligament only, as in Ant. rosacea, their opposed faces being marked by a vertical 

 articular ridge (PL VII. figs. 2 b, 3 a, 5 b, 6 a, i). In almost all of these species which have 

 more than ten arms, the first and second segments beyond every axillary are united by 

 ligament only, just like the second and third radials 5 . Thus in Ant. Savignii every third 

 segment, so long as the division lasts, is an axillary, and the first and second segments 

 beyond each axillary are united by ligament only. But in Ant.palmata only two segments 

 follow each bifurcation, the second of which is again axillary ; it is nevertheless united 

 to the first one by ligaments only. I have found these same two conditions to occur 

 together in Act. polymorpha (PI. II. fig. 8), in which the normal number of segments 

 between every two points of division is three (PI. II. figs. 7, 9, 10), of which the third 

 is axillary Avith a syzygiuin, as in Ant. Savignii, while the second is united to the first by 

 ligament only. In exceptional instances, however (PL II. figs. 8, 11), the second segment 

 may be axillary, and united to the first by hgaments only, as in Ant. palmata. 



In every case, after the division has ceased, the union of the first and second brachials 



1 There are three Comatula in the ' Challenger ' collection which answer to this description. In two of them the 

 first and second distichals and the first and second brachials are united by syzygies, like the second and third radials. 

 But in the third species there is a curious exception to the rule. The rays may divide eight times : and in the primary 

 divisions there are three distichal joints, the first two of which are united by ligaments and not by syzygy. But in 

 all the subsequent divisions the first two joints beyond each axillary form a syz3'gy, like the second and third radials. 



" This agreement between the mode of union of the second and third radials, and of the first and second brachials 

 respectively, is seen also in Encrinus moniliformis, in which these segments are united by syzygia as in P. MiiUeri. 

 See ' Fetref. Germ.' Taf. liv. figs. F, G. The same is the case in Rliizocrinus (Sars, ' Crinoides vivants," pp. 1-3, 22). 



3 P>au des Pentaermus, p. 30, and Taf. ii. fig. 8. 



4 This does not appear, however, to be always the case ; for iliiller described the syzygium as uniting the first and 

 second arm-segments in the specimen examined by him, while I have found the same to be tho case in a specimen of 

 this species contained in the Zoological Museum of the University of Wiirzburg, in which there is certainly no 

 syzygial union between the second and third radials. In Pentacrinus the opposed faces of two elements, which are 

 united by a syzygium, are simple, and not radially striated as in Antedon. Sars has found this to be the case in 

 Bhizoorinus also ; but in its predecessor, Apioerinus obeonicus, Goldfuss, the radial striation of the syzygial surfaces 

 is very distinct (Petref. Germ. Taf. lvii. fig. 5). 



5 The ' Challenger ' collection includes two very abnormal species which present a singular exception to the rule. 

 The rays divide three times ; and the first two segments (distichals) of each of the ten primary arms are united by 

 ligaments only, like the second and third radials. So far the rule holds good ; but with the next arm-division there 

 is a new point of departure. The third or axillary distichal bears the secondary arms, which consist of one axillary 

 segment only. But this segment is itself primitively double, i. c. it consists of two parts united by a syzygy ; and 

 the first joints of each of the ultimate arms borne by tis axillary a gree with it in being syzygial segments. 



