MONOGRAPH OF THE EXISTING CRINOIDS. 575 



TIIATJMATOMETRA NUTRIX. 

 Figs. 1232-1237, pi. 38. 



Dr. Th. Mortensen has given a short account, to be followed later by a detailed 

 description, of the young of this curious viviparous species. 



When ready to attach themselves the larvae do not leave the marsupium, but 

 become affixed to its inner wall, at any point from near the mouth to the bottom, 

 with the calyx and proximal portion of the column protruding through the open- 

 ing. In this position they remain throughout their whole development until they 

 are ready to detach themselves from the stalk. 



In the older stages Morteasen found the distal end of the column, which lacks 

 the terminal stem plate, regularly attached to the second or third pinnular, which 

 is laterally broadened to accommodate it; in one case the column was attached to 

 the articular surface between the third and fourth pinnulars, both of which 

 were broadened. 



The young pentacrinoids are very small, but in the course of their develop- 

 ment they grow to a very considerable size. 



The large pentacrinoids attached to the pinnules of the small mother give the 

 latter a very curious appearance. 



Sometimes two pentacrinoids are attached to the same pinnule. In one case 

 these were in about the same stage of development, but in another one was a nearly 

 full grown pentacrinoid with five cirri, while in the other the arms were just be- 

 ginning to develop. This indicates that an egg may undergo development in the 

 ovary while a pentacrinoid protrudes through the marsupial opening. 



From the number of pentacrinoids found it is apparent that generally only 

 one, and rarely two, eggs at a time are developing in the marsupium. 



None of the fully grown pentacrinoids have more than one or two arms in 

 place, but the cirri are well preserved and show that at least 10 are developed before 

 the young crinoid detaches itself from the stalk. 



Figures given by Mortensen show that at the stage in which one or two 

 brachials beyond the IBr, (axillaries) are present the column consists of five or six 

 short columnals, without a terminal stem plate. 



A large pentacrinoid has the column composed of 10 columnals and a centro- 

 dorsal but without a terminal stem plate. The three columnals immediately 

 beneath the centrodorsal are short and discoidal, progressively decreasing in 

 diameter ; the next is about as long as broad ; the seventh-ninth beyond the centro- 

 dorsal are the longest, though they are not more than half again as long as broad. 



The centrodorsal as shown bears 7 cirri, 5 of which, radial in position, are large 

 and composed of 10 segments, while the others, interradial in position, are rudi- 

 mentary. 



7THAUMATOMF.TRA, SP. 

 Fig. 1214, pi. 34. 



On October 18, 1873, off Tristan da Cunha (lat. 37 21' 00" S., long. 12 21' 30" 

 W.), in 1,000 fathoms, the Challenger dredged three pentacrinoid larvae, but no 

 corresponding adults. 



