C UiAMOCRINUS DIOMED E 75 



in the Revision of the Echini,) and yel the pentagonal shape is one Car- 

 penter insists upon. Sec also Love*n's figures in bis Etudes, Plates XVII. 

 and XXI. 



We may now examine the apical system of the young stages of some 

 of the species of modern Cidaridae, to see if ii throws any lighl upon the 

 probable way in which their anal system assumed its present structure, and 

 incidentally how this corresponds to the SO called archaic type of apical 

 system of the Salenidse and other Echinidae. 



In the young of a Dorocidaris papillata (Plate XXIX. Fig. 8) we find 

 the interradial plates forming a (dosed ring, in the outside angle of which 

 are placed the radial plates. The anal system is covered by five large 

 plates projecting radially into the anal angles of adjoining interradial 

 plates. We may assume that this is not an unusual structure among the 

 young Cidaridae, as Doderlein has figured the young of Goniocidaris bise- 

 rialis (Fig. 3) and of Leiocidaris imperialis (Fig. 4) in stages but slightly 

 older, and in which in one case one. and in the other two, additional anal 

 plates had been formed to separate two of the radially placed plates, and 

 form the beginning of an outer row of ten or more anal plates. 



The youngest Cidaridae figured by Doderlein* also show this disposition 

 very clearly. See his Plate V. Fig. 8, and Plate IX. Fig 8 a, which show 

 the apical system of a young Goniocidaris biserialis, 3 mm. in diameter, 

 and of a small specimen of Leiocidaris imperialis. 4 mm. in diameter. New 

 plates are formed, at first intercalated laterally, between the original five 

 plates. The original five plates are readily recognized in the Cidaridae. 

 I would, in addition to the figures I have already quoted, add the following 

 figures of Doderlein, in which this is very apparent : Plate II. Fig. 1, 

 Stereocidaris grandis ; Plate III. Figs. 6, 11, S. japonica; Plate IV. Figs. 

 11, 12, Porocidaris gracilis; Plate V. Figs. 3, 10, 16, Goniocidaris biserialis; 

 Plate X. Figs. 4, 12, Eucidaris japonica. 



This mode of intercalation of new plates, forcing their way from the 

 interior to the periphery of the anal system, is continued as long as the 

 anal system increases in size, and we can easily trace it even in the figures 

 here given of the apical system of Goniocidaris geranioides (Plate XXIX. 

 Fig. 6), of Dorocidaris Blakei (Fig. 11), of Porocidaris Sharreri (Fig. 5); 

 and we see the same process going on in the formation of the second ring 

 of anal plates in Dorocidaris Bartletti (Fig. 12). 



* Die Japanischen Seeigel. I., Stuttgart, 1887. 



