14 



CATALOGUE OF THE BLASTOIDEA. 



us denote this by the letter A, and call the others B, C, D, E respectively, following 

 the hand of a watch or the spiral twist of the coiled intestine. B and C are there- 

 fore the rays of the left or east side of the disk, while D and E are on the right or 

 west ; so that the anal interradius is between C and D. Viewed from the dorsal 

 side these positions are of course reversed, and the rays B, C, D, E follow one 

 another against the watch-hand. If the same rule be applied to the Blastoids 

 (Fig. I., B), we find that the small basal lies in the left anterior interradius between 

 the rays A and B ; while the suture between the two large basals coincides with the 

 right posterior radius or D. Let us call the small basal x, and the other two y and z 

 respectively, y lying under ray C, and z under E, as shown in the accompanying 

 diagram (Fig. I., B). Comparing the Blastoid with Platycrinus, we may then express 

 the difference between the two types by saying that in the former the basal x is in 

 the interradius A-B, while in the latter it is in A-E (Fig. I., A). 



Among the many species of Platycrinus exceptions will doubtless be found to this 

 rule, but so far as the Blastoids are concerned we have only met with one aberrant 

 individual. It belongs to the common type Pentremites Godoni, and represents an 

 altogether new arrangement of the basals, differing both from that of the ordinary 

 Blastoid and from that of Platycrinus. In the former, as we have just seen, allies in 

 the interradius A-B, and in the latter in A-E; while in the aberrant individual of 

 P. Godoni, x corresponds with the interradius D-E, an arrangement which, so far as 

 we know, has not hitherto been met with in any Pelmatozoon, with the exception of 

 the recent genus Ilyocrinus l . 



Fig. II. 



duns 



Diagrams to show the relations of the dorsal and ventral axes in Ekutherocrinus Cassedayi 



(adapted from Shumard and Yandell). 



The lettering as in Fig. I. A, dorsal, and 15, ventral aspect. 



With the above facts before us, we may now endeavour to determine the position 

 of the radial axis in the asymmetrical AstrocriniiUe. Taking Elcutherocrinus as the 



1 Zool. C'hall. Exp. part xxxii. 1>>1. p. 'J'S.i. 



