REPORT ON THE CRINOIDEA. 219 



the cup and the arms cannot be better described than in the words of Sir Wyville 

 Thomson/ "The second tier consists of five radials, which are thin, broad, and spade- 

 shaped, with a slight blunt ridge running up the centre and ending in a narrow 

 articulating surface for an almost cylindrical first brachial. 



"The arms are five in number, they consist of long cylindrical joints deeply grooved 

 within, and intersected by syzygial junctions. The first three joints in each arm consist 

 each of two parts separated by a syzygy; the third joint bears at its distal end an arti- 

 culating facet from which a pinnule springs. The fourth arm-joint is intersected by two 

 syzygies, and thus consists of three parts; and so do all the succeeding joints; and each 

 joint gives ofi" a pinnule from its distal end, the pinnules arising from either side of the 

 arm alternately. The proximal pinnules are very long, running on nearly to the end of 

 the arm; and the succeeding pinnules are gradually shorter, all of them, however, running 

 out nearly to the end of the arm, so that distally the ends of the five arms and the ends 

 of all the pinnules meet nearly on a level." In all cases the first pinnule is on the left 

 side of the arm. I can say nothing as to the total number of pinnules, the longest arm 

 remaining having six of these appendages on eacli side. Owing to the large size of the 

 pinnules in comparison with the arms, the epizygal joints to which they are articulated 

 have the appearance rather of axillaries than of ordinary brachials. This is also the case 

 in Khizocrinus, but to a less extent (PL IX. figs. 4, 5). But as these appendages are 

 simple and contain the genital glands like the pinnules of other Crinoids, they are 

 undoubtedly of that nature, and must not be regarded as branches of the arms. 



The mouth is protected by a very perfect, five-sided pyramid of triangular oral plates, 

 the outer surfaces of which are deeply hollowed along the median line (PI. VI, figs. 1-4), 

 while the inner surface slopes away rapidly on either side from a strong central keel 

 (PL VI. fig. 5). Sir Wyville Thomson described it as marked with deep impressions for 

 the insertion of muscles ; but I believe him to have been mistaken in this point. There 

 is no trace whatever of any such muscles l)eing attached to the inner surface of the oral 

 plates in the mutilated specimen represented in fig. 5 ; while the orals of Rhizocrinus and 

 of the Pentacrinoid larva of Comatula are certainly not so provided with muscles, and 

 there are no a priori reasons whatever for invoking their presence in Hyocrinus. 



About half the diameter of the disk is occupied by the oral pyi'amid which covers up 

 the central mouth. Between its base and the edge of the cup there is a pavement of 

 closely set, thin plates belonging to the anambulacral system, which have no regularity either 

 of form or of arrangement. Some of these extend upwards on to the anal tube, which is 

 situated near the edge of the disk in one of the interradial spaces. As in Rhizocrinus 

 the oral plates are pierced by the ciliated water-pores which lead downwards into the 

 body-cavity (PL Vc. fig. 6, ivp). But the pores are more numerous than in Rhizocnnus, 

 which has only one in each oral plate. In both the specimens of Hyocrinus which I have 



^Journ. Linn. Soc Lond. (ZooL), vol. xiii. p. 52. 



