178 ROBERT TRACY JACKSON ON ECHINI. 



The lantern of Strongylocentrotus drobachiensis is taken up first as a type of structure, 

 because opportunity offered to study living material and abundant fresh specimens for dis- 

 section. The structure has not been published in this type excepting in so far as Lov6n (1892) 

 described the jaws and muscles in the young; the study has, therefore, interest from making 

 known the structure in a common type and adds some features not previously recorded. In 

 Strongylocentrotus there are five keeled teeth lying free about the mouth. The tooth is held 

 in place by the two half-pyramids which ventrally enclose it closely by an inward extension 

 of the base of the lateral wings (Plate 5, fig. 5). The tooth rests against the dental slide (s. d.) 

 on the inner face of the pyramids. On each side the dental slide is produced dorsally as a guid- 

 ing or styloid process, which in face view is seen projecting above the base of the foramen mag- 

 num (Plate 5, figs. 4, 7). After traversing freely the space of the foramen magnum, in this type 

 the tooth rests against two guiding processes, the crests of the epiphyses (Plate 5, figs. 2-5, 7, 

 9). The tooth then curves inward, takes a second sharp curve on itself and in the same plane, 

 and terminates in a free point in the dental capsule (Plate 5, fig. G). From the mouth to a 

 point just above the crests the tooth is calcified; beyond this point to the proximal tip the tooth 

 is soft and pulpy and easily destroyed. In area 1 of Plate 5, fig. 9, the soft part of the tooth 

 has been removed, whereas it is in place in areas 2 and 3. The calcified part is that usually 

 shown in figures of lanterns, and is the only portion preserved in fossils, as in Archaeocidaris 

 (Plate 12, figs. 4-8) and Pholidechinus (Plate 27, figs. 4-6). An entire tooth spread out fiat is 

 shown in Plate 5, fig. 8; it is seen that at the dorsal, young or growing portion of the tooth 

 it is grooved, but, passing ventrally, the character of the keel is soon taken on. That is, the 

 young, last added portion of the tooth as a localized stage in development is grooved, as is the 

 whole tooth in the Aulodonta, Cidaroida, and Palaeozoic forms; whereas the older or earlier 

 formed portion of the tooth has taken on the full specific and subordinal character of a keel. 

 The pyramids are each composed of two pieces joined by a median or pyramidal suture (Plate 

 5, figs. 4, 7, p. s.), and each piece is spoken of as a half-pyramid. Dorsally the pyramid in- 

 cludes a wide angle, the foramen magnum, which in this and allied species is rather deep. On 

 the inner side at the base (Plate 5, fig. 5) the wings of the pyramid curve around so as to embrace 

 the tooth tightly at its point of exit. Each half-pyramid consists of a peripheral portion, the 

 outer face of which gives rise to two muscles, and the inner bears one of the two dental slides 

 which support each tooth, and a lateral wing which is strongly ridged horizontally for attach- 

 ment of the interpyramidal muscles. The dorsal face of the half-pyramid, seen only when the 

 epiphysis is removed (Plate 5, figs. 9, 10), presents a series of pits of considerable size and depth. 

 These pits have apparently been overlooked by previous observers. They are important, as 

 they are characteristic of the order of the Centrechinoida, but are not found in other orders 

 of Echini. In 100 specimens of Strongylocentrotus drobachiensis from Frenchman's Bay, 

 Maine, examined for this character, pits exist in all the pyramids, but there is considerable 

 variation as regards the pattern and depth of the pits; occasionally they are very shallow. 



