MONOGRAPH OF THE EXISTING CRINOIDS. 587 



ently are soon resorbed, though it is possible that they develop into the cluster 

 of interradial plates often found between the IBr x in the adults. 



Interradials incorporated in the radial circlet are found in Promachocrinus 

 kerguelensig and similarly in the species of Tliaumatocrinus, although the penta- 

 crinoids of this latter genus are not known. 



In Promachocnnus the interradials appear first, after the radial circlet has 

 been completely closed for some time, as narrow triangles on the distal apices of the 

 basals. Eapidly extending themselves distally, they soon come to separate the 

 radials as very narrow interradial plates, resting upon the truncated distal angles 

 of the basals, which widen rapidly and from the middle of the distal edge give 

 off a IBrj and IBr 2 followed by brachials, which exactly resemble the same plates 

 following the radials, but are much smaller. 



In postpentacrinoid growth these interradials and the arms which they bear 

 grow somewhat more rapidly than the radials and the postradial series beyond 

 them, so that when the full size is reached the animal possesses 10 radials and 

 20 arms which are indistinguishable one from the other. 



There can be no doubt that the five additional radials and the five additional 

 arms of T 'haumatocrinus are formed in exactly the same way, and it is likewise 

 probable that in six-rayed specimens of other species the sixth ray (almost invari- 

 ably inserted behind the left posterior) has the same origin. 



It is a very curious fact that in the young of Promachocrinus often no, or 

 only one, interradial is developed, so that five and six rayed specimens are very 

 common, though apparently these never attain full size. 



It is not absolutely certain that the interradials of Promachocnnus and of 

 T haumatocrinus are homologous with those of the young of Comactinia, Com.issia, 

 and Antedon; but the so-called anal x in certain fossil types, from which a series 

 of ossicles arises, appears to be a connecting link between them. 



Arms. The arms do not differ appreciably in structure in the different types. 

 Almost from the first, however, the relative slenderness or robustness of the post- 

 radial series of the adults is forecasted. 



Pinnules. The pinnules from their first appearance essentially resemble the 

 definitive form. 



The distal pinnules, from the ninth to the twelfth or even the fourteenth 

 brachials onward, are the first to appear, followed by the pinnule on the second 

 brachial (PJ and, considerably later, by the pinnules on the intervening brachials. 



The relation between the development of the pinnules and the development 

 of the cirri has already been discussed. 



In some forms, as in Antedon, the pinnule on the second brachial develops 

 when only two or three pairs of distal pinnules are present. In others, as in 

 Hathrometra, it develops much later. 



In the following species all the pinnules are present at the time of detachment 

 from the larval column : 



Heliometra glacialis. Hathrometra prolixa. 



ProtnacJiocnnus kerguelensis. Hathrometra tenella. 



Hathrometra sarsii. 



