26 STRUCTURE OF THE CYSTIDE^. 



Fig. G. One of the rays of Rhodocrinus bursa (Phillips). 



In this figure r, r, r, are the three primary radials ; s, s, the 

 secondaries ; a, o, the ambulacral orifices ; b, the five basal plates ; 

 ^, two of the sub-radials ; and a, what appears to be the place of a 

 pair of arms. In some of the specimens this featm'e is exhibited 

 so prominently that it strongly impresses the observer with the idea 

 of two tiers of brachial appendages. It may be however that there 

 were projecting from this part of the vault a set of large spines 

 corresponding in numbers with the arms. Rhodocrimis differs in no 

 respect from Thysanocrinus (Hall), provided we still depend for 

 generic characters altogether on the structure of the cup below the 

 point where the arms become free. But if the form of the vault 

 be taken into account, then the English genus is different from the 

 American. 



The vault in R. bursa rises above the ambulacral orifices, and in 

 fact projects a little outward over them, so that they penetrate into 

 the side of the cup, below the margin, instead of being placed 

 immediately above and inside of the margin, which, from the 

 position of the arms, must be their place in Thysanocrinus. 



4. Actinocrinus rugosus. — For the structure of this interesting 

 species I am indebted to Mr. Huxley, and the figures given in the 

 following page were drawn from a spacimen in his possession. 



In this species the plates are very thick, and the ventral side 

 rises dome-shaped above the point of attachment of the arms, so 

 that the ambulacral orifices are rather nearer the bottom than the 

 top of the body. The proboscis is excentric, large, and not perpen- 

 dicular, but projecting obliquely, so that when the arms were closed 

 its apex probably was thrust out between two of them on one side. 

 The mouth appears to have been closed by a number of small plates, 

 which were no doubt so connected by an extensible membrane as 



