22 ]ME. P. II. CAEPENTEE ON TJIE GENUS ACTINOMETEA. 



This view, although uuquestiouahly correct in the case of the Articulate Crinoids 

 {Comatula, Pentacrinus, &c.), is, as Roemer has pointed out, beset with some difllculties 

 in its application to the fossil Tessellata ; and Schultze ^ accordingly reverted to the 

 original view of Miiller, saying, " Die Arme (hrachia) beginneu uuvcriinderlich da, wo 

 eine doutliche Gelenkfacette cines fcsten Kelchstuckes, ihrcn Ursprung anzeigt." In 

 describing the divisions of the arms, he speaks of the brachial axillaries of the first, 

 second, and third order, Avithout giving them any special names. These are perhaps 

 scarcely necessary when the number of segments between each division varies so much 

 in different specimens and in different arms of the same specimen as it does in many 

 fossil Crinoids, and in Pentacrhms. Among the Comaiulce, however, the number and 

 character of the segments between the successive divisions of the arms exhibit variations 

 which are to a certain extent constant in different species, and thus give us the means of 

 classifying them into larger or smaller groups. 



Miiller has availed himself of this character to a certain extent in the scheme which 

 he gives" of a classification of the Gomatulce according to the jiresencc or absence of 

 syzygia in the various hrachial axillaries ; hut though, in his descriptions of the different 

 species, he furnishes the material for carrying this classilication much further, and for 

 separating species which, in his scheme, stand very near to one another, he never made 

 any use of it, simply classifying the Conialulce in the groups which he had constituted, 

 according to the number of their arms — 10, 20, 40, or more. Under these circumstances 

 he would have heen puzzled where to place Act. polymorpha, in which I have found the 

 number of arms to vary from 13 to 39. 



(§ 10) It has been already stated that the arms proper of Comatula hegin from the 

 radial axillaries. In many cases they are united by perisome as far as their second or 

 third division ; and in Act. muUiJida this perisome contains numerous small calcareous 

 plates, which render the union of the arms with one another and with the calyx some- 

 what firmer than usual ; hut they are never so united as to be immovable, as their 

 various segments are connected with one another, except, of course, at the syzygia, by 

 muscles and ligaments. There is one point about the nature of this union Avhich has 

 not, I think, received sufficient attention ; and as it shows clearly that the arms of 

 Comatula and Fentacr'mus begin from the radial axillaries, it is worth considering here. 

 It is this : the first and second segments beyond every axillary, whether radial or 

 hrachial, are nearly always united together in the same manner as the second and third 

 (axillary) radials. 



Thus, for example, in Act. Solaris, and in the forms allied to it, the second and thu-d 

 radials are united by a syzygy. The same is the case Avith the first and second brachials. 

 In Aiit. rosacea, and in the various species which are closely allied to it, there are no 

 muscles between the second and third radials ; but their opposed articular faces 

 present a vertical and not a transverse ridge, and are so united by ligament that the 

 two segments are only capable of a lateral movement upon one another, and cannot take 

 part in any movements of flexion or extension, in which they act as a single segment 

 only. The first and second brachials are united in precisely the same manner. 



' Monographic dcr Kchinodenncn dos Eiflerkalks, (Wicn, ISUG) p. 5. 

 • Gattung Comatula, p. 11, 



