466 ADDITIONAL NOTES 



The disc is elongated when the four ocular plates, the anterior, lateral, and posterior, are 

 longitudinally on the same line with the ovarials. The single ovarial plate is sometimes 

 absent, as in the sub-compact disc ; it is oftener, however, represented by one or many 

 small, irregular, and imperforate complementary pieces, as in Hyhodypus and Collyrites, 



The discs which I have discovered in Galerojpijgus agariciformis and G. caudatus 

 undoubtedly belong to the sub-compact group, and justifies M. Cotteau in removing 

 them into the genus he has established for their reception.* 



ECHINOBRISSIDtE. 

 Clypeus Plotii, Klein. PI. XLIII, fig. 4 a, b, Supplement to page 364. 



Previous to the publication of the figures and description of Clypeus Plotii in this 

 Monograph, no notice had been taken by former authors that the small tubercles of this 

 urchin were perforated ; the specimen I figured Avas supposed by some to have been 

 exceptional rather than typical, as several accurate observers had failed to verify Mr. Bone's 

 figures. Accordingly I exposed several specimens of Clypeus Plotii on my garden-wall 

 during two winters, and effectually weathered the surface of their tests ; by this process 

 I have ascertained, that all the tubercles on the inter-ambulacra, and likewise on the 

 ambulacra, including even the minute granules ranged on the edge of the zonal septa, 

 are perforated. I have in fig. 4 a represented a portion of the upper surface of this 

 urchin, of the natural size, and in fig. 4 h given a magnified view of one of the plates thereof; 

 the bosses of the tubercles, with their deep-encircling areolae, and the perforation of the 

 summits, are well represented in this drawing, together with the miliary granules which are 

 freely scattered over the surface of the test. The form and structure of the apical disc are 

 likewise well seen in fig. 4 a ; the elements of this compact disc are covered by the 

 madreporiform body, which in this species extends over the surface of all the genital and 

 ocular plates, the only indication of these bodies being the five orbits at the summits of the 

 ambulacra and the four openings of the genital ducts opposite the inter-ambulacra ; the 

 spongy structure of the madreporiform body is likewise beautifully exempHfied in this 

 weathered specimen. 



* For ample details on the genus Galeropygns the reader is referred to M. Cotteau's excellent memoir 

 on that new genus, in the 'Bulletin Soc. Geo], de France,' 2me scrie, t. xvi, p. 2S9. 



