650 MESSRS. RITCHIE AND MciNTOSH ON [June 16, 



than the acloral, but the portion facing the ambulacrum is con- 

 siderably deeper than that remote from it (text-fig. 142). 



In the plates of the ambulacral areas thei'e are more frequent 

 departures from the visual form. Fully-developed ambulacral 

 plates are formed by the union of small pore-plates, each bearing 

 a single pore-pair. In Echinus esculentths three of these primitive 

 plates formed near the apical area, under the shelter of the 

 oculars, are compressed, by the formation of new plates, to form 

 a compound triad, the ordinary plate of the ambulacral area. 

 Even in the fused plate the original pore-plates can be distin- 

 guished by shallow .boundary grooves ; and we are following the 

 usual terminology in designating the two outer plates, which 

 are bounded on one side by the interambulacral area and on the 

 other by the zigzag intra -ambulacral suture, the adoral and aboral 

 primaries ; while the median plate touching the interambulacral 

 area but failing to reach the zigzag suture in its own area, is 

 known as a demi-plate. The three pore-pairs in a compound 

 plate are arranged, not in a single vertical series, but lie in three 

 distinct longitudes. These details of plate structure have been 

 recounted in order to facilitate reference to the abnormalities 

 which occur, and which consist, for the greater part, of an 

 imperfect complement, or an incomplete fusion of the primitive 

 plates which ordinarily go to the formation of a compound ambu- 

 lacral plate. 



In the posterior series, a, of the right posterior ambulacral 

 area, I (text-fig. 138) the twenty-eighth plate, numbered from the 



Text-fig. 138. 



b 



I. 



Abnoemalities in Ambulaceal Aeeas or Echinus esculentus. 



Eoman numerals beneath tlie figures indicate the ambulacral area in which the 

 abnormalities occur. Arabic numerals alongside the figures indicate the 

 numbers of the plates, reckoned from the peristome. a and h, series in 

 ambulacral areas. In text-fig. 142 two interambulacral plates are included, 

 and are numbered according to their area, series, and position in series. 



peristome, consists of only two complete primaries, a demi-plate 

 being lacking. It is succeeded by a solitary demi-plate, perhaps 

 the remains of the aboral of twenty-eight, the place of which may 



i^^^^V' 



