190 ROBERT TRACY JACKSON ON ECHINI. 



radial in position, its two halves united by suture and embracing tlie tooth (text-fig. 210). 

 This is the usual way of considering it, and undoubtedly the best for description of the lantern 

 itself. Or a pyramid may be considered as radial in position, the two halves united by the 

 interpyramidal muscle being the unit, and the halves thus considered are the point of origin 

 of all the lantern muscles that are inserted in one radial coronal element (text-figs. 218, 219). 

 As stated (pp. 43, 62), the corona of a sea-urchin maj- be considered as made up of five ele- 

 mental radial parts, each of which is developed in relation to one of the ocular plates, as it is at 

 this point that all new coronal plates originate. Any one of these five parts consists of an 

 ambulacrum and on each side a half-interambulacriun, as these originate on the ventral border 

 of the overlying ocular plate. The genitals are omitted from this consideration as coronal plates 

 do not originate in relation to them and one may be absent (spatangoids) without affecting 

 the corona (p. 167.) This relation is shown in Palaeechinus drawn from the exterior (text-fig. 

 217), and Eucidaris drawn from the interior (text-fig. 218). The other half-interambulacra 

 not figured would be associated with the next adjacent ambulacra and ocular plates on either 

 side. The compass, brace, and two half-pyramids face this ideal radial area, as in text-figs. 

 218-220, but the teeth would be interradial in position. In Eucidaris (text-fig. 218) the radial 

 compass muscles, protractors, and retractors pass to the apophyses of the two half-interambula- 

 cra on either side. In Centrechinus (text-fig. 219) the radial compass muscles and protractors, 

 as in Eucidaris, pass to the apophyses of the two associated lialf-interambulacra, but the re- 

 tractors have been transferred to the two auricles that develop on the ambulacrum (compare 

 text-fig. 225). This divides the sea-urchin more naturally than to consider the ambulacrum 

 as one and the interambulacrum as a second and independent part. 



The perignathic girdle has been studied by Duncan (1885a) and critically by Loven (1892). 

 I have little to add to their studies except a correlation with fossils and a general summing up 

 of relations. In text-figs. 221-236 is shown the base of two ambulacral and two interambulacral 

 areas of the corona, seen from within, with one set of those lantern muscles that are inserted 

 on the base of the corona. Thus the structure is shown both with and without the associated 

 muscles in the leading orders and representative families of Echini. Bather (1909) has shown 

 that in the Permian Miocidaris keyserlingi there are apophyses (text-fig. 238 bis) as in the 

 Recent cidarids, but in other Palaeozoic genera no perignathic girdle has been found in any 

 type, and all the evidence goes to prove that no such structure existed. I have seen as actual 

 tests or internal molds the interior of the basicoronal plates in Lepidocentrus, Hyattechinus, 

 Palaeechinus, Lovenechinus and Perischodomus, and in none of these was there a trace of any 

 supports for the attachments of lantern muscles. Moreover, as Loven (1892) showed in very 

 young Cidaris, the muscles are attached directly to the base of the primordial interambulacral 

 plates, no apophyses having at that stage developed. From the structure of the lantern in 

 Palaeozoic genera (Plate 27) it is evident that muscles existed, and it is assumed that the 



