ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 181 



character in the young. Loven (1892, p. 13, Plate 4) described the lantern of a young Strongy- 

 locentrotus drobachiensis only 1.2 mm. in diameter. At this stage the pyramids are wide-angled, 

 so that the whole lantern is broadly inclined instead of being nearly perpendicular, as in the 

 adult; also the sides of the half-pyramids bearing the interpyramidal muscles are curved instead 

 of being in a nearly vertical plane as in the adult, and as a result the interpyramidal muscles 

 are relatively longer than in the adult. In these several characters the lantern of young 

 Strongylocentrotus point for point agrees with the typical Palaeozoic lantern as seen in Archaeo- 

 cidaris (Plate 12, figs. 4-6) and Pholidechinus (Plate 27, figs. 4-6). The same agreement with 

 the Palaeozoic characters is seen in the lantern of young Goniocidaris (Plate 2, fig. 17). 



Having considered the lantern and muscles of Strongylocentrotus, representative lanterns 

 of Echini will be taken up in the sequence of their structural differentiation. Of Bothriocidaris 

 nothing is known but the tips of the teeth, or pyramids. They lie opposite the ambulacra 

 (Plate 1, fig. 1). That they occupy this anomalous position is without explanation unless by 

 an accidental twist they have come to assume it. It is the most ambiguous feature of this 

 important type. 



I have been so fortunate as to secure for study some excellent lanterns of Palaeozoic genera. 

 These are described under their several species, but are briefly considered here. The essential 

 character of the lantern in the Echinocystoida and Perischoechinoida is represented by 

 Archaeocidaris (Plate 12) and Pholidechinus (Plate 27) ; other genera as far as known present 

 only slight differences. Of other genera it is known more or less completely in Palaeodiscus, 

 Oligoporus (Plate 50, figs. 11, 12), Melonechinus (Plate 56, figs. 9, 10), Lepidesthes (Plate 68, 

 figs. 9-14), Pholidocidaris (Plate 74, figs. 2, 6, 7), and Meekechinus (Plate 76, fig. 7). In the 

 Palaeozoic genera the teeth are grooved, and when in place, extend slightly above the base of 

 the foramen magnum (Pholidechinus, text-fig. 207, Plate 27, figs. 5, 6). While the teeth are 

 usually obtusely acuminate orally, as in Pholidechinus and Lepidesthes (Plate 68, fig. 9) , in one 

 genus, Meekechinus (Plate 76, fig. 7), the teeth are deeply serrate distallj^ a unique character. 

 The pyramids are wide-angled in outline and as a result the lantern is inclined at an angle of 

 about 45 degrees instead of being nearly perpendicular as in the adults of most living regular 

 Echini; also the lateral wings of the pyramids are curved, indicating quite long interpyra- 

 midal muscles (Plate 27, figs. 4, 6). In these characters the Palaeozoic lantern approaches 

 closely that in the young of modern types (Goniocidaris, Plate 2, fig. 17), and also the adult 

 of Recent echinothuriids (Plate 2, figs. 19-21). The foramen magnum is moderately deep 

 (Archaeocidaris, text-fig. 208, Plate 12, fig. 4). It appears very shallow in Pholidechinus 

 (Plate 27, fig. 6), but this is in part due to the position in which the specimen was drawn. 

 The lateral wings of the pyramids present a corrugated surface for attachment of interpyra- 

 midal muscles (Plate 12, fig. 8; Plate 13, fig. 13; Plate 68, fig. 12). The dorsal surface of 

 the half-pyramids, seen when the epiphyses are removed, is smooth, as in the Cidaroida 



