ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 183 



ing erect, and the foramen magnum is extremely shallow, the pyramidal suture extending nearly 

 the length of the outer face (Plate 2, figs. 8, 10). The pyramid has the usual corrugations on 

 the lateral wings (Plate 2, fig. 7). When the epiphyses are removed, the dorsal faces of the 

 pyramids present a smooth surface (Plate 2, fig. 9, area II). This is an important character 

 in which it resembles Palaeozoic genera, but differs from the Centrechinoida in which pits 

 typically occur in this area. The epiphyses are narrow, extending only slightly beyond the 

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

 tubercles for articulation with the brace (Plate 2, figs. 7, 9, 14, 15). The brace is of the usual 

 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 compass is composed 

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

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

 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, so as to show 

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

 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 shallow 

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



In the Centrechinoida the structure of the lantern is extremely interesting and affords 

 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 series of pits 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, (avXds, a groove, and oSovs, 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 sub- 

 order or Stirodonta (a-relpa, a keel, and oSous, 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 (/ca/iapa, anything with an arched covering, and oSovs, 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 setosus (text-fig. 211, text-figs. 219, 220, p. 191) the teeth 



