202 PBOF. p. MARTIIi DUNCAK ON THE 



In the third specimen (fig. 43) the second plates of both zones 

 come to the edge, and the first plates of both zones are crowded 

 towards the median line and are narrow and tall. This is a com- 

 plete departure from the type of Cidaris and Echinus. In fact, 

 the very unsymmetrical method of junction of the plates 1 and 2 

 in fig. 43 seems to indicate that the first plates might be lost 

 altogether (fig. 49). The sutures between the ridges and pro- 

 cesses are easily separated owing to their lamellary condition ; 

 and it can be seen, on the face of the junction of the ridge aud 

 the process, that there are three plates in the ridge of that side 

 which come to the interradial sutural face (figs. 44 and 47). The 

 part of the ridge which is produced towards the peristome is seen 

 in fig. 45. 



JEchinothrix Desori, Peters. — This species has exceedingly 

 broad and very low ridges ; and in most instances there is a 

 median projection on a ridge at the free curved edge. The ridges 

 are formed by three plates in each zone, instead of one iu addition 

 to the usual single and double plate of the diff"erent zones (fig. 48). 

 At the edge there is a triangular and small plate with its suture 

 at the median line, slightly departing from the vertical ; and the 

 other suture is between this plate (1), zone a, and the succeeding 

 low but broad plate 2, But this last plate is oblique, and reaches 

 from the free edge near the process to the median suture. Then 

 comes plate '3, also oblique, and being the usual large plate of 

 the zone. In the other zone, i (fig. 38), the first plate occupies the 

 whole of the upper edge of the ridge on its side ; and plate 2 is 

 the usual low plate of the zone, and plate 3 is the usual large 

 plate. 



The ridge is, then, mainly composed of three plates, two on one 

 side of the median line and one on the other. 



There is nothing which requires notice with regard to the 

 processes, except that they are expanded above and have a large 

 opening (fig. 46). 



But the bases and the ambulacral plates close by are well 

 worthy of study, for the peculiar distribution of the pairs 

 enables the direction of the plates to be distinguished. Benzene 

 also assists, so that the relation of the suture between the process 

 and the ridge, and its continuation between the ambulacrum and 

 the interradium, can be seen, and the connection of this long line 

 of junction with the ambulacral plates (poriferous part) can be 

 made out. 



