ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 177 



and fill the periproctal area. In irregular Echini periproctal plates are relatively numerous 

 and always fill the area completely. As clearly shown by Mortensen (1907, Plate 13) in Brisaster 

 (Schizaster) fragilis, the periproct in the very young individuals is within the apical disc, as in 

 regular Echini. It early travels out and soon assumes its adult position in the posterior inter- 

 ambulacrum. 



In regular Echini the periproct is typically enclosed by a ring of genital, or genital and 

 ocular plates. In the Echinothuriidae, on account of the separation of genitals and oculars, 

 the periproct maj^ come in contact with the interambulacra (text-fig. 170, p. 149). As a rela- 

 tively rare variation, a similar condition may exist in other regular Echini, as Sirongylocentrotus 

 gibbosus (text-fig. 156, p. 145), S. drobachiensis (Plate 5, fig. 15), and S. lividus (Plate 6, fig. 5). 

 In such types the young interambulacral plates develop in contact with the oculars as usual, 

 and the periproct has no relation with the interambulacra excepting that of mechanical contact 

 (pp. G3, 110). In the Exocycloida the periproct lies in interambulacrum 5; it is therefore 

 separated from the ocular and genital plates of the apical disc and completely surrounded by 

 interambulacral plates. 



The Aristotle's Lantern and Perignathic Girdle. 



Comparatively little has been pubhshed in regard to the Aristotle's lantern in Palaeozoic 

 Echini. Trautschold (1868) figured complete lanterns in Archaeocidaris rossica, SoUas (1899a) 

 figured a lantern in Palaeodiscus, and Meek and Worthen (1873) in Lepidocidaris in part. 

 The lantern and its muscles in living Echini have been worked out quite fully by a number 

 of authors, the most important being Valentin (1841), A. Agassiz (1872-'74, 1904, 1908, 1909), 

 T. H. Stewart (1861), and Lov4n (1892). 



The jaws and muscles are often very inadequately or incorrectly given in text-books. 

 For comparison with fossils, a lantern with soft parts was worked out iij Strongylocentrotus 

 (Plate 5) as a basis for comparison. The lantern of this genus and a number of others not 

 previously described were studied, and are given more or less fully in the following pages. The 

 perignathic girdle might properly be described with the lantern in the several types, but it 

 seemed best to treat it separately (pp. 189-198). 



It is believed that the structure of the lantern is of great value in systematic classification, 

 and that the structure of its several parts presents characters that are of ordinal or subordinal 

 value. As Dr. Mortensen pointed out (1904, p. 54), the structure of the teeth, keeled or un- 

 keeled, is "a very important character, though it has hitherto received very little attention." 

 Besides the teeth there are other features of value. Briefly stated, the essential points are: 

 teeth grooved or keeled ; epiphyses narrow, or wide and united by suture ; the top of the pyramids, 

 as seen when the epiphyses are removed, a smooth floor, or pitted ; foramen magnum deep, or 

 shallow; angle of outline of the lantern depressed or erect; compasses present or absent. 



