186 ROBERT TRACY JACKSON ON ECHINI. 



possessing jaws. The brace is of the usual shape, and the compasses are strongly arched, 

 curving over the outer border of the braces (Plate 2, fig. 19). The muscles of Phormosoma 

 are of the usual number in lanterns of regular Echini, but in addition extra muscles are given 

 off from the auricles radially. These muscles (text-fig. 226, p. 193) are very delicate and may 

 fairly be called radial peristomal and radial somatic muscles, as they extend like tent-ropes 

 from the auricles radially to these two areas. They have been described in Asthenosoma as 

 longitudinal muscles (Lang, 1896, p. 471), but this name seems inappropriate. These muscles 

 apparently cause movements of the highly flexible test of the corona and peristome (p. 195). 



The Aulodonta have the primitive structure of grooved teeth and narrow epiphyses seen 

 in the Echinocystoida, Perischoechinoida and Cidaroida. With this they have the deep foramen 

 magnum, prominent styloid processes, and especially pits in the top of the pyramids character- 

 istic of the Centrechinoida. 



Stirodonta. — Salenia is a type of much interest. Duncan (1889a) gives the characters 

 of the lantern, but I believe they have not been shown in detail or figured. The teeth are 

 keeled and supported by the dental slides and styloid processes as usual (Plate 4, figs. 3, 4). 

 The pyramids are rather wide-angled, have a deep foramen magnum (Plate 4, fig. 3) , and when 

 the epiphyses are removed, show pits in their dorsal faces (Plate 4, fig. 4, area I). The epi- 

 physes are very narrow, only slightly more than covering the dorsal faces of the half-pyramids 

 (Plate 4, fig. 4, area I). The foramen magnum is therefore open dorsally. The braces and 

 compasses have the usual shape. The protractor and radial compass muscles pass from their 

 origin in the half-pyramids and compasses to the basicoronal plates of the half-interambulacra 

 associated with the area in which they originate (Plate 4, figs. 4, 5). The retractor muscles 

 pass from the half-pyramids to auricles developed from the ambulacra. In Salenia the keeled 

 teeth, deep foramen magnum, pits in the dorsal face of the pyramids, and the auricles are all 

 characters which unquestionably locate this type as one of the Centrechinoida and equally 

 separate it from the Cidaroida with which group Salenia was formerly associated (p. 195). 



The lantern of Stomopneustcs variolaris, I believe, has not been described. It is erect, 

 with teeth keeled, epiphyses narrow, not meeting in suture over the foramen magnum (Plate 4, 

 figs. 8-10). The dorsal ends of the half-pyramids extend inward over the foramen and at their 

 free ends are produced ventrally as spurs which meet and give support to the teeth. The 

 function of these pyramidal spurs is similar to that of the crests developed on the epiphyses 

 of the Camarodonta (text-fig. 213), but they cannot be homologized with them as they arise 

 from a different structural part. I believe this is a feature which has not hitherto been noted 

 in the lantern of Echini. Stomopneustcs has been placed in the Echinometridae, but the 

 structure of the lantern would locate it in this suborder, and it seems that it may be safely 

 followed as a basis of classification. In Glyptocidaris (Phymosoma) crenulare, the only living 

 representative of its family, the Phymosomatidae, the structure of the lantern is as in Stom- 



