ZOOLOGICAL (!l \i; ACTEB8. 119 



in some Crinoids, but we know of no member of either group in which their 

 arrangement is at. all like that which occurs in the Blastoids. In this class thej arc 



restricted to the radial and interradial plates, where they lie with their slits parallel 

 to the ambulacra, and, except in the genus Codaster, there are always five interradial 

 pairs of hydrospire-groups. No Cystid whatever presents anything like this regular 

 distribution of the hydrospires, which often extend down on to the basals, and even 

 on to the under-basals, when such are present. 



Considered as a whole, the Blastoids have the most regularly constructed calyx of 

 any Pelmatozoa, and in fact of any Echinoderms. Under-basals maj occur in all the 

 brachiate forms, while the basals of Crinoids may vary in number from two to five, 

 and the development of the interradials varies extremely among the different groups. 



The primitively simple apical system of an Urchin eventually undergoes very 

 considerable modifications, while that of Stellerids is complicated by the great 

 development of a number of additional plates. In the Blastoids, however, there is 

 only one exception to the number of thirteen plates in the calyx, viz. Elceacrinus, 

 which has the posterior deltoid divided into two by the anal plate; while, except in the 

 three genera Eleutherocrinus, Astrocrinus, and Pentepkyllum, the contour of the 

 calyx is always perfectly symmetrical, consisting as it does of five equal and similar 

 radials, and five deltoids of the same nature 1 . 



Taking then all these characters together, we find that the Blastoids constitute a 

 remarkably compact group, which is pretty clearly marked off from the other Pel- 

 matozoa; and we cannot at all agree with Wachsmuth and Springer-, who say that 

 the Crinoidea, "but especially the Blastoidea, are linked together with the Cystidea 

 by such easy transitions, that among the earlier types it is difficult to draw any clear 

 line of demarcation. We are unable to point out a single character that is not found 

 exceptionally in one of the other groups." They give this as a reason for refusing 

 the rank of a class to either Crinoids, Cystids, or Blastoids. 



The relation between Crinoids and Cystids will be discussed by one of us elsewhere ; 

 but there is no difficulty in mentioning characters of the Blastoidea which are not as 

 yet known to occur in either of the other groups. Such for example are the perforate 

 lancet-plate, the regular limitation of the hydrospires to the radial and interradial 

 plates, with their slits parallel to the ambulacra, both points of very considerable 

 importance, as well in a morphological as in a physiological aspect. What Cystid 

 has such a regular calyx as a Blastoid ! Cryptoerinus cerasus seems to have the same 

 symmetry in its base, but the rest of its calyx is constructed on a totally different 

 plan from that of a Blastoid. The Lower Silurian Cystoblastus 3 has two rings of 



1 The anal deltoid is different from the rest in Trooslocrinus (see pp. 36, 112), and slightly so in Grana- 

 tocrinus neglectus, M. it \V. 



2 ' Revision of the Paloeocrinoidea,' Part III. 1S85, p. 75. 



3 See Volborth, " Ueber Achradoeystites und Cystoblastus, zwei neue Crinoideeu-Gattungen." Mem. Acad. 

 Imp. Sci. St. Pe'tersbourg, 1870, vii.° se'r. torn. xvi. Mem. 2, p. 11. 



