ECHINID^i. 549 
lacral pores transposed, that they may be enclosed by the sub-anal faSciole. 
Another organo-genetical law discovered by Loven, and traced minutely 
through all divisions, is the remarkable asymmetry (especially distinct in the 
peristomial portion of the ambulacra of the triviiim), pointing perhaps to 
a yet unknown relation between the axes of the young Sea-Urchin and 
its plutean larva. In connection with this investigation, the transforma- 
tions undergone by the elements and organs of the shell during growth, 
are traced and shown to be subjected as strictly to unfailing laws. 
There is a profound diversity between the Cidaridce and all other 
EchinidcR in the manner in which the peristomial ambulacral plates ar§ 
in the former successively detached from the ambulacra and form the 
imbricate covering of the peristomial membrane. The palaeozbic EchU 
nidm are not yet sufficiently known to be tested as to the laws of asymme- 
try, &c., established for the mesozoic and recent types ; but in some re- 
cent Spatangidos, a few traces of an (originally ?) imbricated arrangement 
of the plates may still be found. Highly suggestive as is the analysis 
of the part played by the dislocation of the vent in the various types 
and genera, through the course of geological times, showing the path 
that should be followed in a rational investigation of the successive 
modification of types, it yields in general morphological importance to 
the demonstration of the homology of the “ apical system ” of the Echi- , 
nid(B with the “calyx” of the CrinoidcB\ e.g., the “ocular” plates corre- . 
spending to the “ radialia,” the “ genital ” plates to the “ costalia,” the 
central (“ sur-anal ”) disc to the “ basale” in Marswpites. The homology 
is equally close between the “ calyx ” of the Sea-Urchins and the dorsal 
skeleton of the young Starfish, before the appearance of the perisomatic 
skeleton ; even the madreporite and the vent making their appearance in 
exactly corresponding places. The two great fundamental divisions of 
the Echinidet are the E. dentata and edentata. 
W. Keeping proposes the following arrangement of the Echinoidea 
(J. G. Soc. xxxii. p. 40). 
Perischoechinida (more than 2 rows of plates in the inter-ambulacral 
areas) — 
Tessellata (plates non-imbricating). 
(plates imbricating). 
Echinida (2 rows of plates in each area) — 
Echinothuridee (plates imbricating, test flexible). 
Stereodermata (plates not imbricating, test rigid). 
Endocyclica (vent surrounded by the apical plates). 
Exocyclica (vent not surrounded by the apical plates). 
In Wyville Thomson’s account (14) of. the EcJiinidm collected during 
the deep-sea expeditions of 1868-70, from the Faero Islands to the ^traits 
of Gibraltar, and from 600-2435 fathoms \_cf. Zool. Rec ix. p. 440, as to 
the general results], full characteristics are given of the families Cidaridm 
and Echinothuridee, and of the genera Cidaris, Porocidaris, Phormosoma^ 
Calveria, Neolampas, and Pourtalesia. The following species are de- 
scribed and copiously illustrated in 13 excellent plates: — Cidaris papiU 
lata, Leske (= hystrix, Blv.), p. 722, pi. lix. figs. 1-13; affinis, Phil. 
