ON THE ECHINOIDEA. 465 



Galeropygus AGARiciroRMis, Forbes. Supplement, PI, XLIT, fig. 2 « and fig. 3, 



pages 292-95. 



Galekopygtjs agaeiciformis. Cotteau, Bulletin Soc. Geol. de France, 2me serie, tom. xvi, 



p. 289. 



Altliougli many hundreds of specimens of this mxhin had passed through ray hands 

 when I described the species, still I had not then seen any traces of the apical disc ; I was, 

 therefore, unable to give any opinion upon M, Cotteau's proposal to separate into a dis- 

 tinct genus, under the name Galeropygus, those Hyhodypi which possessed a sub-compact 

 and not an elongated disc. Very lately, however, I met with two specimens of this 

 urchin which possessed portions of the disc in situ, and these form the subjects of 

 figs. 2 and 3 of the Supplemental PI. XLII. 



The four ovarial plates, Avhich are of a rhomboidal figure, are arranged in a crescentic 

 form around the concave, anterior opening of the round, discal aperture ; the right antero- 

 lateral plate is the largest, and supports the madreporiform body ; the plates are small, 

 and externally present acute angles, which are inserted into the V-shaped notches of the 

 inter-ambulacral segments of the discal opening ; the foramina for the ovarial tubes are 

 at the extreme point of the angle, and in some almost marginal. Four of the ocular plates 

 are very small, and intercalated between the angles of the ovarial plates ; the left postero- 

 lateral is larger than the others, and wedged between the two left lateral ovarials, and all 

 the orbits are distinctly marginal (fig. 3). The posterior part of this singular structure is 

 absent, and it does not appear in what manner the single ovarial was articulated with the 

 others, nor how the membrane of the vent, with its anal plates, was united to the test. 



M. Cotteau first proposed the separation of Hybodypus disculus, Ag., into the 

 genus Galeropygus, from observing the difference which the disc of that urchin presented 

 when compared with the true type form of the genus, Hybodypus yibberulus, Ag. 



The apical discs of the Echinoidea exoctclica may be arranged, as M. Cotteau 

 observes, into three groups — 1st, compact ; "^^fmh-compact ; and 3d, elongated. 



The disc is said to be compact when the ovarial plates form a circle around the madre- 

 poriform body, and when the five small ocular plates are intercalated between the angles 

 formed by the ovarial plates, as in Holectypus, Clypeus, Galerites, and JEcMnoconus. 



The disc is sub-compact when the three anterior ocular plates are intercalated between the 

 angles of the ovarial plates, whilst the two posterior ocular plates are longitudinally on the 

 same line as the postero-lateral ovarials ; sometimes the single plate is altogether wanting ; 

 but it is oftener represented by two or three small, complementary, imperforate pieces, 

 Avhich reach the madreporiform body. This disposition of the plates gives the disc a 

 sub-circular form, such as is seen in Pyrina and Galeropygus. 



