MONOGRAPH OF THE EXISTING CKINOIDS. 553 



about 30. Except for being much elongated, the brachials have attained approxi- 

 mately the perfected form. The dorsal surface of the first 8 or 10 brachials is 

 thickly beset with fine spines, and the distal border is armed with much longer 

 spines. This latter character persists and becomes accentuated distally. The 

 syzygies are distinguishable from the muscular articulations, but the brachials 

 on either side of them have not become appreciably shortened. Very large and 

 conspicuous rounded covering plates, between which are the large and conspicuous 

 dark brown sacculi, border the ambulacral grooves on either side. There are five 

 sacculi and covering plates for each three brachials. 



Long and slender pinnules are developed from the twelfth or thirteenth 

 brachials onward, and the pinnule on the second brachial (Pj) is present. The 

 pinnules do not differ essentially from those of the adult, but the component 

 segments are much more elongated. The third-fifth segments bear seven very 

 large and conspicuous covering plates, which considerably exceed in height the 

 lateral diameter of the segments bearing them. Large dark-brown sacculi occur 

 between the covering plates. 



The interradial arms are as yet very small and rudimentary, with the tips 

 turned inward. They reach only to the distal border of the first brachial. 



The character of the disk can not be made out, as it is hidden by the arms; 

 but orals appear still to be present. 



The essential features in the development of the pentacrinoids of Promacho- 

 crinus kerguelensis may be summarized as follows: 



Coluvm. — Broadly speaking, the column is similar to that of the species 

 of Hathrometra and Heliometra. It reaches a great length and is composed, when 

 fully grown, of more than 65 segments, of which a usually considerable but very 

 variable number of the proximal are short and discoidal; but the increase in 

 diameter of these toward the calyx is never very marked. 



The interesting feature in regard to the column is the occurrence just beneath 

 the centrodorsal of either a rosette of five wedge-shaped plates or a single plate 

 sharply divided into five sections, in the middle of which the tapered point of 

 the centrodorsal rests. The right rounded outer ends of these plates or sections of 

 a plate are interradial in position. 



Notches in the middle of the proximal borders of the basals in many speci- 

 mens in which the centrodorsal has not as yet developed, which are evidently 

 to accommodate the interradial processes just described and which indicate that 

 the columnal immediately beneath the basals is the one which is later found beneath 

 the centrodorsal, suggest that the centrodorsal is not formed from the lateral en- 

 largement and thickening of the most proximal columnal as in Antedon, but 

 that it originates as a very small circular plate, entirely excluded from external 

 view, situated between the center of the basal ring and the topmost columnal, 

 and that this plate increases in diameter only through the addition of broader 

 and broader layers between itself and the basal ring, so that it assumes from the 

 first the form of an inverted cone the base of which is applied to the basal ring, 

 while the slightlj' truncated tip rests in the middle of the f>olumnal beneath it. 



