Prof. S. Loven on the Structure of the Echinoidea. 443 



base of Marsupites and the vertex of the Salenidte is to be 

 found in the elevated ridges which in both unite the middle 

 points of the plates ; and the strongly developed vertical plates 

 of the Salenidse scarcely present any "sculpture" which 

 does not recur in a similar form in the Crinoidea. 



Now, since the central disk in the young Echinidee and in 

 the SalenidiB is to be regarded as homologous with that of 

 MarsujnteSj the five plates which embrace it, and which here 

 are called vertical plates, but have hitherto borne the name of 

 genital plates, are ipso facto to be interpreted as basal pieces 

 (basalia), and the " eye-plates " in their reentrant angles as 

 radial pieces (radialia). A calyx is present in its essential 

 parts, homologous, by its position at the pole opposite to the 

 mouth, its constitution, and its structure, with that of the Cri- 

 noidea. But since the Echinoidea are free animals which 

 turn their mouth towards the surface whence it takes its food, 

 the calyx comes to be the vertex of the dermal skeleton instead 

 of its base. It receives the newly formed plates of the corona, 

 the basalia meeting the growing ends of the interradia, and 

 the radialia those of the ambulacra. In the Echinidee which 

 have their anal aperture where the peduncle of the Crinoidea 

 is attached, the calyx is normal and recognizable in its form ; 

 in the Clypeastridae it is most frequently entirely penetrated 

 by the madreporite, which effaces the sutures of the pieces ; 

 and in the irregular forms with an elongated antero-posterior 

 axis and a developed bivium {Echmoneus, Cassidulida?, and 

 Spatangidte) it becomes entirely abnormal, and, in the Colly- 

 ritidje, during the Jurassic and Cretaceous periods, was broken 

 up, so that the two radialia which meet the bivium were sepa- 

 rated from it by the perisome. But it is not absent from any 

 form of Echinoidea. 



The investigations which are here communicated will, it is 

 hoped, speedily appear in a more detailed form, illustrated by 

 a selection from numerous figures carefully prepared by M. 

 A. M. Westergren. 



EXPLANATION OF PLATE XIV. 



Fiff. 1. A young Toxopneustes drobachensis of 4 millims., spread out from 

 the peristome. I., II., III., IV., V., ambulacra; 1, 2, 3, 4, 5, 

 interradia. In the middle the mouth with the teeth ; around 

 this, in the buccal membrane, ten free pore-plates, two for each 

 ambulacrum, of which the five which lie before I. a, II. a, III. 6, 

 IV. a, V. b are perceptibly larger than the others. Peristomial 

 plates I. rt-V. b and 1. 6-V. a are united by straight lines ; and 

 by this means two pentagons are inscribed in the circular stoma, 

 symmetrical only in relation to the diameter a a>. The plates 

 of the vertex kept together, upon interradium 2 (where the madre- 

 porite has its place), with the central disk, d, c. The inner circle, 



31* 



