ARISTOTLK'S LVNTKHN AM) I'l.UK-N \TIIK (JIIUH.K 183 



ing erect, and the foramen magnum is extremely shallow, t l.r pyramidal suture extending m-arly 

 the length of the outer face (Plate 2, figs. 8, 10). Th<- pyramid ha- the n-nal corrugation- ..., 

 the lateral wings (Plate 2, fig. 7). When the epiphyses are remove,!, the do,-al faces of tin- 

 pyramids present a smooth surface (Plate 2, fig. <), area II). This is an important cha. 

 in which it resembles Palaeozoic genera, but differs from the ( Vnirerhinoida j,, which p j t , 

 typically occur in this area. The epiphyses are narrow, extending only .-lightly l>eyond the 

 upper face of the half-pyramids. Each presents a glenoid cavity and internal ami external 

 tubercles for articulation with the brace (Plate 2, figs. 7, 9, 14, 15). The hraee i- of the n-ual 

 shape with rather strongly marked condyles and external and internal foramina for articulation 

 with the glenoid cavities and tubercles of the epiphyses. Each ossicle of the compa- i- eompooed 

 of two pieces and is strongly arched, with a bifid outer end (Plate 2, fig. 12). A lantern i- -h..wn 

 on Plate 2, fig. 9, drawn to give the full characters from the dorsal view. In area I all the part- 

 are in place; in V the compass is removed to show the brace; in IV the compass and brace are 

 removed to show the epiphyses; and in area II the epiphyses also are removed. -<> a- to .-how 

 the dorsal face of the half-pyramids. This figure may be compared with similar druwing- 

 of Phormosoma (Plate 2, fig. 20), Salenia (Plate 4, fig. 4), and Strongylocentrotus (Plate 5, 

 fig. 9). These figures bring out essential differences in their several separate groups. The 

 lantern of Eucidaris as shown has the character of its order and is essentially similar to that of 

 the Perischoechinoida. The chief differences are the angles of the pyramid and the .-hallow 

 foramen magnum. These differences, however, are bridged by the young. 



In the Centrechinoida the structure of the lantern is extremely interesting and afford- 

 data for grouping the families in three distinct suborders on the basis of the characters of the 

 teeth and the epiphyses. The order as a whole differs from other orders in that the dorsal face 

 of the half-pyramids (seen when the epiphyses are removed) shows a .-erie- of pit- instead of 

 plane surfaces. The order is characterized by having a deep foramen magnum instead of a 

 very shallow one, as in the Cidaroida, or a moderately deep one, as in the Perischoechinoida. 



The first suborder, which I would call the Aulodonta, (avXdj, a groove, and 68ous, tooth) is 

 characterized by grooved teeth and narrow epiphyses, not meeting in suture over the foramen 

 magnum (Centrechinus, text-fig. 211; Phormosoma, Plate 2, figs. 19-21). The second sul>- 

 order or Stirodonta (crreipa, a keel, and 68ou's, tooth) is characterized by keeled teeth and 

 narrow epiphyses, not meeting in suture over the foramen magnum (Salenia, Plate 4, figs. 

 3, 4; Glyptocidaris; Stomopneustes, Plate 4, figs. 8-10; Arbacia, text-fig. 212). The third 

 suborder or Camarodonta (Ka^dpa, anything with an arched covering, and 0801/5, tooth) is 

 characterized by keeled teeth and wide epiphyses, meeting in suture over the foramen magnum 

 (Tripneustes, text-fig. 213; Strongylocentrotus, Plate 5, figs. 1-12). A more detailed consider- 

 ation of these suborders and their characters follows. 



Aulodonta. In Centrechinus seiosus (text-fig. 211, text-figs. 219, 220, p. 191) the teeth 



