ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 185 



2n. Cenlrechinus selosus (Leske). Bermuda. R. T. J. Coll., 788. X 1.6. Top of pyrainid pitted, tooth grooved, 

 epiphyses narrow, foramen magnum deep (compare text-figs. 219, 220, p. 191). 



212. Arhacia punclulata (Lamarck). Woods Hole, Massachusetts. R. T. J. Coll., S83. X 1. Top of pyramid 

 pitted, tooth keeled, epiphyses narrow, foramen magnum deep. 



213. Tripneusles esculenlus (Leske). Florida. R. T. J. Coll., 884. X 2.4. Top of pyramid i)illcd, tooth keeled, 

 epiphyses wide, meeting in suture with crests supporting tooth, foramen magnum deep (compare Plate 5, figs. 1-12) . 



214. Clypeasler rosaceus {lAnwi). Bahamas. R. T.J. Coll., 885. X 1.6. Top of pyramid smooth, pyramids later- 

 ally flaring, epiphyses narrow, compass wanting, foramen magnum very shallow. 



215. The same. Tooth keeled, in side and face views and in section. 



216. The same. Revolved outward to show ventral tip of tooth. 



Text-figs. 207-209, pyramid wide-angled, lantern inclined; 210-213, pyramid narrow-angled, lantern erect; 214, 

 pyramid flaring, lantern procumbent. 



Lettering: 6, brace; c,compass; cr., crest; cp., epiph3'sis, top view and in place; /, foramen magnum; p, top of pyramid; 

 St., styloid process; (, tooth. 



are grooved and continue calcified high above the base of the foramen magnum. They are 

 supported by the dental slides, as usual, and the long styloid processes, typical of the order, 

 are a prominent feature in face view. The pyramids are narrow-angled, with a deep foramen 

 magnum, and the upper borders of the pyramids curve inward over the space of the foramen 

 magnum and a slight distance beyond the epiphyses. The top of the pyramids has deep pits 

 seen when the epiphyses are removed (text-fig. 211JB). The epiphyses are narrow, not quite 

 covering the dorsal face of the half-pyramids. The brace and compass are as usual except 

 that the compass may be outwardly rounded instead of bifid as in most Echini. In text-fig. 

 219 (p. 191), the protractor and radial compass muscles are shown passing from the lantern 

 to the two half-interambulacra associated with ambulacrum III and the retractor muscles 

 passing to the auricles that originate on ambulacrum III. 



The genus Phormosoma is most interesting as representing the remarkable Echinothuriidae. 

 Mr. Agassiz (1904, Plate 47) figured the structure of the lantern in Echinosoma hispidum (A. 

 Agassiz), and I show it in another species, Phormosoma placenta. The teeth are grooved, 

 narrowly acuminate distally (Plate 2, figs. 19-21). They are supported by the dental slides 

 and styloid processes as usual. The pyramids are very wide-angled with long interpyramidal 

 muscles. As the angles are wide, the lantern is inclined instead of nearly perpendicular, the 

 usual character of the suborder, and in this respect it is closely like that in the adult of Palaeo- 

 zoic and the young of Recent species. This is best seen by comparing the dorsal view of Phormo- 

 soma (Plate 2, fig. 20) with that of Pholidechinus (Plate 27, fig. 5). The foramen magnum 

 is deep and widely curving, a very distinctive character. The dorsal face of the pyramids has 

 the pitting seen in all the order when the epiphyses are removed, as shown in Plate 2, fig. 20, 

 area II. In side view (Plate 2, fig. 21) the lateral wing of the pyramid is corrugated as usual, 

 and is strongly bowed on its outer line. The epiphysis is very narrow with the usual glenoid 

 cavity (Plate 2, figs. 19, 21, and 20, area IV). Duncan (1889a, p. 40) is mistaken in saying of 

 the Echinothuriidae that epiphyses are wanting; they doubtless exist in all regular Echini 



