56 SVEN LOVEN, ECHINOLOGICA. 



convergent, in tlie Clypeaster they are wide-open, flg. 57, the 

 inner wings being very short, and divergent. The figure 

 formed by the snpra-alveolar crests, fig. 144, is not circular 

 nor pentagonal, but obovate, contracted posteriorly, and of" a 

 narrow compass, its longest radius, the 5, being within half 

 the length of the pyramid. If the distance between the middle 

 of the rotnlse I and V is taken = 1, that between the rotulai 

 I and IV, IV and V, is =^ l,i2, and the distance between II 

 and III, IV and III — 1,035. The increase of the hmgitudinal 

 and antero-posterior axis of the body, and the bilateral sym- 

 nietry on either side of it, the 1 and 4 being broader in 

 proportion internally than the 2, 3 and 5, are thus expressed 

 in the general form of the dental system. If the specimen 

 is tnrned and the month examined the same proportions are 

 found to reappear likewise in tlie size of the teeth, Fl. VII, 

 fig. .52, 60, 61, Fl. XI, fig. 149, of whieh the 5 is the 

 1 argest and most prominent, then the 2 and 3, while the 

 1 and 4 are the smallest. The points of 5, 2 and 3 meet to- 

 gether and keep the teeth 1 and 4 apart from one another. 

 When a half-pyramid of Clypeaster reticnlatus L. and a cor- 

 responding one of, for instance, the Echinometra Lncunter 

 L.' are placed side by side, with the inside of the alveole 

 and the symphysis up, the support of the tooth will be found 

 to agree very closely in both. There is in either the corres- 

 ponding half of the nearly vertical, linear, incurved slide for 

 the back of the tooth, s. de. fig. 52, 57, 60, ending upward in 

 a styloid prqjection, prominent in the Echinometra, sessile in 

 the Clypeaster. 'But there the resemblance ceases. In the 

 former the entire area of the symphysis is vertical, narrow 

 and lunate, its external outline being regularly arcuated, and 

 it ends adorally under an acute angle. In the Clypeaster, 

 fig. 60, the same area is narrow and pointed, also, in its upper 

 part, only gradually widening downward, for about 0,4 3 of 

 the length of the dental slide. But at that point its upper 

 outline is strongly diverted outwards at an obtuse angle, 

 while the under outline is also directed outward, and the 

 two lines, joining at the rounded external extremity, circum- 

 scribe an expanded, tongue-shaped surface, symph.; about half 

 as long again as the dental slide is high. This large area of 

 the symphysis answers to the narrow and simply lunate one 

 ' Wood engraving p. 53, fig. 3, c. 



