216 ROBERT TRACY JACKSON ON ECHINI. 



suborder, the epiphyses are wide and meet in a medium suture over the foramen magnum (text- 

 fig. 213, p. 184; Plate 5, fig. 2). The epiphyses further develop crests that serve as supports 

 for the upper part of the tooth. These features are known in no other Echini and express the 

 fullest differentiation attained in the lantern of regular Echini (p. 187). 



In the Temnopleuridae, the circular form of the test, the ambulacral plates of three ele- 

 ments each, with the curious pitting or sculpturing developed on at least the upper part of the 

 test are the most distinctive features. The peristome is mostly naked except for the primordial 

 ambulacral plates. Of Pleurechinus doederleini, Mortensen (1904, p. 78) says that there are 

 only five, instead of ten, buccal plates, a unique character. This is a very remarkable departure, 

 as the ambulacrum, both in the peristome and in the corona of Echini, so constantly consists 

 of a right and a left half for each area. The ocular plates are mostly exsert in modern types, 

 a primitive character, but one or more are said to be insert (Duncan, 1889) in some fossil types. 

 In the Cretaceous Glyphocyphus radiatus all oculars are insert, p. 117. This family on account 

 of its peculiarities represents probably a side branch rather than the basal series of its suborder. 



The Echinidae is a large family including many genera and species; the test is circular 

 or slightly pentagonal; ambulacral plates have three elements each. Plates with two elements 

 each occur in Echinus affinis, but this is exceptional, and they exist dorsally only (p. 118). 

 The peristome has ten primordial ambulacral plates of moderate size (Loven, 1892, Plate 12, 

 fig. 156) with more or fewer large or small to granular non-ambulacral plates. Exceptionally 

 there is a dense covering of non-ambulacral plates (Toxopneustes, text-fig. 57, p. 84). The 

 oculars may be all exsert, or ocular I, or very frequently I, V, or rarely I, V, IV may be insert 

 as a species character; as progressive variants, oculars I, V, IV, II, or all plates may be 

 insert. The range of variation is wide in many species, but with rare exceptions oculars reach 

 the periproct in the sequence I, V, IV, II, III (text-figs. 115-127; pp. 160, 161). The 

 commonest aberrant variant for oculars is for I, V, II to be insert, a combination seen only 

 in this family, the Strongylocentrotidae, Stomopneustidae, and Saleniidae (p. 164). In Gymne- 

 chinus (p. 120) typically an abnormal arrangement of oculars occurs as described. 



The periproct is plated with many larger or smaller plates, and in adults frequently a 

 somewhat larger plate, which is apparently the suranal, lies against or near genital 3 (text-fig. 

 122, p. 122). The size which renders this plate distinguishable is subject to much individual 

 variation. It is markedly developed in some, as Toxopneustes variegatus. Of the perignathic 

 girdle the auricles are moderately developed and commonly meet in an arch over the ambula- 

 rum; apophyses are usually only slightly developed. 



The Strongylocentrotidae (Gregory, 1900, p. 313) includes the large genus Strongylocen- 

 trotus and a few others. The test is circular as in the Echinidae, but, with the exception of 

 some species of Echinostrephus, there are from four to ten elements to an ambulacral plate. 

 As in the Echinidae, oculars may be all exsert or otherwise reach the periproct in the sequence 



