VI. Pyrgocystis. 



1915 55 



According to the definitions given by me in 1899 and 1900 

 ("Treatise on Zoology," pp. 207-208), it would go more readily 

 with the Agelacrinidaa ; but in those definitions the differences in 

 the subvective skeleton were not sufficiently taken into account. 



In Studies I, II, IV, and V, the subvective skeleton of four 

 Edrioasterid genera has been described in detail, and shown to consist 

 of floor-plates and cover-plates, both disposed in alternating series, 

 with pores between the floor-plates. 



The evidence adduced by C. F. Eoemer (1851), F. B. Meek (1873), 

 Miller & Faber (1892), 0. Jaekel (1899), J. M. Clarke (1901), and 

 W. K. Spencer (1904) proves that in Agelacrinus (sensu lato) and 

 Hemicystis among Agelacrinidse, the subvective skeleton likewise 

 consists of floor-plates and cover-plates, but that the floor-plates 

 form a single series stretching right across the groove, and show 

 no trace of pores. There is also a difference in the cover-plates, 

 for normally in these two genera, as also in Cystaster and Streptaster, 

 they are boot-shaped, with the sole of the boot adoral (= proximal) 

 and the toe of the boot adraedian, and in consequence of this shape 

 they cannot close in the groove so completely as do the symmetrical 

 cover-plates of the Edrioasteridse. In some species of Agelacrinus, 

 as shown in J. Hall's well-known figure of A. Cincinnati emis (1871), 

 there are small additional cover-plates along the median line, much 

 as may occur exceptionally in Edrioaster (Study IV, text-fig. 2). 

 These additional plates, however, involve no departure from the 

 essential plan of either group. 



These two plans do not, I believe, exhaust the actual constructions 

 employed in the Edrioasteroidea. Setting aside Cyathocystis and 

 Stromatocystis, in which the structure is still not fully known, we 

 may note Lebetodiscus (Study III), in which the cover-plates appear 

 to be minute and to lie over the sutures between the floor-plates. 

 The specimen figured by Jaekel (1899, Stammesges. d. Pelmatoz., 

 pi. ii, fig. 2) as Agelacrinus Diclzsoni Billings, presents notches along 

 the sides of its supposed cover-plates, and in other specimens I have 

 observed these notches to be very distinct and regular : so that in 

 such forms, it has seemed to me, there may have been minute cover- 

 plates resting in the notches, and the larger visible plates may really 

 be floor-plates. 



The specimens of Pyrgocystis, so far as one can judge from the 

 obscure evidence, do not display either cover-plates or floor-plates 

 of the typical Edrioasterid or Agelacrinid plan. It is, however, 

 conceivable that the plates observed correspond to the notched plates 

 of the third plan of structure. Appearances that may possibly 

 represent notches have been mentioned in the description of P. grayce, 

 and indications that the plates may perhaps not be cover-plates have 

 been given in the description of P. sardesoni. 



Those species which seem to me to have this supposed third plan of 

 subvective structure, appear in all other points more closely allied 

 to the Agelacrinidae than to the Edrioasteridse. In those same 

 points it is with the Agelacrinidse that Pyrgocystis also agrees. 

 Therefore this genus may for the present be left in that Family. 

 Although Steganoblastus is the genus of Edrioasteroidea that has the 



