188 ROBERT TRACY JACKSON ON ECHINI. 



The keeled teeth as in the Stirodonta are a mechanical advance over grooved teeth (text-figs. 

 207-213, p. 184). In addition the confluence of the epiphyses (text-fig. 213, p. 184; Plate 5, 

 figs. 2, 5, 9) is a mechanical advance, as, besides supporting the teeth, it makes with the braces 

 a continuous rigid base as fulcra for the action of the interpyramidal muscles, instead of a 

 distinct continuous base as occurs in all lower Echini (text-figs. 207-212, p. 184; Plate 2, figs. 

 19, 20; Plate 27, figs. 5, 6). 



Turning to the irregular Echini, Love"n (1892) and Hawkins (1909) showed that in Discoidea, 

 representing the Holectypina, the pyramids are wide-angled. This is like the character in 

 the young of Recent regular Echini and in the adult of Palaeozoic species, so that in Dis- 

 coidea it may be considered a regressive type of lantern (p. 218). Loven (1892) worked out 

 the lantern and muscles of several of the Clypeastrina with minute care. The lantern in this 

 suborder is highly flattened, broadened, and specialized in character, being very different from 

 the same structure in any regular Echini. A representative portion of a lantern of Clypeaster 

 is shown in text-figs. 214-216 for comparison. The tooth is short and narrow, and the keel, 

 which is relatively very wide (text-fig. 215), makes up a large part of the mass of the tooth. 

 A little depression in the tip of the tooth, represented in section, is probably due to mechanical 

 wear. The keeled teeth of the Clypeastrina indicate a relation of this group to regular Echini 

 with keeled teeth. When the lantern is held in its natural plane (text-fig. 214), only the dorsal 

 tip of the tooth is visible, but if rotated upward (text-fig. 216), the ventral tip is seen as well. 

 The pyramids are broadly triangular and procumbent with a lateral expansion into great wings 

 which are deeply plicate on their outer faces. The median suture exists as usual ; the foramen 

 magnum is extremely shallow, almost a straight line. On the lateral wings of the half-pyramids 

 there are no corrugations, as in regular Echini, for the attachment of interpyramidal muscles, 

 although these are well developed. On the dorsal face of the half-pyramid seen when the 

 epiphysis is removed (text-fig. 2145), there are no pits as in the Centrechinoida, but the sur- 

 face is smooth. The epiphyses are very small and the brace is a small, narrow, block-shaped 

 plate quite different from that of the regular Echini. The compasses are wanting, apparently 

 an ordinal character. 



The Aristotle's lantern is an internal structure and as such is not so likely to be affected 

 by external conditions as the more superficial parts of an urchin. It seems, therefore, that its 

 structure as a complicated mechanism should yield data of classificatory value, binding together 

 the larger groups, and I believe that the facts fully support this view. 



Summing up the characters briefly: in the Palaeozoic Echinocystoida and Perischoechi- 

 noida (text-figs. 207, 208), the lantern is inclined at an angle of about 45 degrees, not erect. 

 The teeth are grooved, pyramids wide-angled, foramen magnum moderately deep, epiphyses 

 narrow, and there are no pits in the top of the pyramids. 



In the ancient Cidaroida (text-fig. 210) the structure is similar, but the lantern is erect, the 

 foramen magnum very shallow, and there are no pits in the top of the pyramids. 



