170 ROBERT TRACY JACKSON ON ECHINI. 



plate lying dorsal to ocular III is apparently the equivalent of the suranal, as in Acrosalenia 

 wiltoni (text-fig. 105), which also has four large plates. In examining some 2,300 specimens of 

 Arbacia punctulata, considerable variation was found -in the periproctal plates, and they varied 

 in adults from two to many (text-figs. 200-205). Two specimens were found with only two 

 plates and many with three. In both of these forms what I take to be the suranal overlaid 

 genital 3, as in Salenia, not ocular III as usual. A number of specimens were found with five 

 plates and several with more than five, one being shown with six and one with thirteen (text- 

 figs. 204, 205) . In one abnormal specimen from Florida (text-fig. 205a) there is one large peri- 

 proctal plate facing ocular III, as usual, and, in addition, some fifty small rounded periproctal 

 plates of a character similar to those usual in the Echinidae. When there are five periproctal 

 plates in Arbacia, as in text-fig. 203, the condition is exactly like that of Habrocidaris (text- 

 fig. 206). In Parasalenia gratiosa A. Agassiz, as shown by Mr. Agassiz (1873, p. 435), there 

 are four periproctal plates, as in Arbacia, but they lie in a different plane so that two are on 

 each side of the antero-posterior axis through III, 5. Of five specimens of Parasalenia gratiosa 

 seen in de Loriol's collection, in the Geneva Museum, four have the typical four plates in the 

 periproct, but one has only three. Variations of periproctal plates in Arbacia dufresni were 

 also noted as described, p. 115. 



In the young of the Echinidae and Strongylocentrotidae a suranal plate fills the periproct, 

 as shown by Mr. Agassiz (1874, p. 732). This is typically developed in young Echinus (Plate 3, 

 fig. 14) and Strongylocentrotus (text-figs. 131-134, p. 129). As the animal grows, the suranal 

 gradually loses its prominence, new plates forming along its right posterior border and the 

 suranal retaining while recognizable a position dorsal to genital 3. In the adult of Strongylo- 

 centrolus drobachiensis a plate is often found dorsal to genital 3 somewhat larger than other 

 periproctal plates (text-figs. 135-139, p. 132). It seems that this may be recognized as the 

 suranal still holding a slight supremacy in size. A similar plate similarly located is frequently 

 recognizable in other genera, as Toxopneustes (text-fig. 122, p. 122) and Echinus (text-fig. 

 115, p. 117), and may fairly be considered as the suranal. There is considerable individual 

 variation, some specimens having no such larger plate at this area and others possessing one. 

 Dr. Mortensen (1911) expresses the view that the suranal is not a primitive structure form- 

 ing an essential part of echinoid morphology, but is a specialized structure found only in 

 certain groups of Echini. I heartily agree with this view. It seems that the suranal is 

 simply one of the plates of the periproct which has attained exceptional prominence and 

 which in youth entirely fills this area in those families in which it is a marked feature (Saleni- 

 idae, Echinidae, Strongylocentrotidae). 



In the adults of the Temnopleuridae, Echinidae, Strongylocentrotidae, and Echinometridae 

 (text-figs. 115-161) the periproctal plates are usually relatively large and solid, more or less 

 numerous, but not definitely limited to a small number (except in Genocidaris and Parasalenia), 



