POBOCIDABIS MILLEET. 27 



of the corona (Pis. 7, fig. 6; 8, fig. -5), in which one or two isolated ambu- 

 lacral plates are situated, the last to become separated from the ambulacral 

 zone. The interambulacral space is covered with numerous, ill-defined, 

 calcareous plates (PI. 7. fig. 6), reminding us of the irregular pavement of 

 the Echinidiv proper, formed of thin plates rather than of the row of solid, 

 interambulacral, buccal plates characterizing the Cidaridse. In other species 

 of Porocidaris (P. elegans, 1 P. Gobosi 2 ) the actinal system is exclusively 

 covered by the buccal ambulacral plates, with the exception of a small 

 area at the base of the interambulacral areas, which may be bare or carry 

 one or more small, ill-defined, calcareous plates. 



It is possible that the extraordinary resorption of the actinal interam- 

 bulacral plates, due to the great growth of the auricles, is the cause of 

 the anomalous development of the buccal plates of this species. It is 

 interesting to compare the figures of the auricles of a specimen of P. Millcri 

 28 mm. in diameter seen facing the auricles from the exterior (PI. 8, fig. 9), 

 from the actinal side (PI. 7, fig. 7), and from the interior (PI. 8, fig. <s') with 

 the corresponding figure of the small auricles of P. Cobosi of a specimen 

 18 mm. in diameter (PI. 11, fig. 4), and in which the buccal plates cover 

 the whole of the actinal system (PI. 11, figs. /-/). 



Small, short-stemmed pedicellariaa are found on all the plates of the 

 abactinal system and in the abactinal part of the ambulacral area. Longer 

 and larger stemmed pedicellaria? are found arranged round the primary 

 tubercles. 



The abactinal system of this species is most prominent, the genital plates 

 appear as if rising above the general level of the test (PI. 6, figs. 3, J t ) 

 both in the males and females; they are readily distinguished even when 

 no larger than 18 or 20 mm. (PI. 7, figs. 3, 4); in larger specimens, 

 probably full-grown, of 28 to 30 mm. the difference in the size of the 

 genital openings is very marked (PI. 7, figs. /, £). The anal system in 

 the larger specimens is usually larger in the males than in the females. 

 In the larger males the occurrence of two or more genital openings in the 

 same plate is not uncommon (PI. 7, figs. 1, 4). The genital openings in 

 the females are placed nearer the interambulacral sutures than in the 

 males. In the youngest specimen examined, 10 mm. in diameter, no 

 genital openings could be traced. The genital plates of the larger speci- 

 mens are most irregular in shape (PL 7, figs. 1-5), the anal system 



1 " Challenger " Echinoidea, PI. Ill, fig. 3. * See PI. 11, figs. 1, 2. 



