AMBULACRA 01" THE RECENT DIADEMATID^. 101 



corresponding sockets may be seen near the ambulacral and 

 median ends of the interradial plates. They are especially 

 visible close to the sutures whicli unite the auricles to the inter- 

 radia. 



Tlie knobs and the sockets are evidently modifications of the 

 parallel lines of laminge, but although their existence cannot be 

 ignored, it cannot be advanced that they contribute to the 

 strength of the suturing. Finally, it appears that very visible 

 grooving and corresponding ridging of tlie horizontal edges of 

 the interambulacral plates occurs when the tubercles are rather 

 close to the adoral edge of a plate. The base of the tubercle 

 forms a kind of overlap, and this is very significant when other 

 Diadematidse are considered, 



IV. Genus Echestothrix, Peters, Monatsh. AJcad. Berlin, 1854, 



p. 101. 



The critical description of tMs genus and its comparison with 

 the other Diadematidse may be read in the ' Revision of the 

 Echini,' by A. Agassiz, p. 413. Both Peters and A. Agassiz 

 state tliat the structure of the ambulacra in this genus differs 

 from that seen in the genera Diadema and Astropyga. The 

 ambulacra differ from those of the last-mentioned genera " in 

 having many vertical rows of very small tubercles." The 

 compound plates are numerous in Echinothrix, and the number 

 of the pairs of pores in a .given vertical space is greater than 

 in Diadema. Consequently there is not the room for large 

 primary and big secondary tubercles in the ambulacra of JEcM- 

 nothrix as there is in Diadema. The trij)lets of EcJiinothrix 

 are close, and the vertical dimension of the combined plates is 

 small. External appearances would lead to the belief that the 

 arrangement of the sutures of the ambulacra is like that of such 

 Triplechinidae as Strongylocentrotus, as drawn by Loven, or that 

 it would resemble the diagram drawn of the sutures of the triplet 

 of Diadema by A. Agassiz ; but a careful examination shows that 

 the plates are arranged after the type of Diadema, as explained 

 in a former page, there being no demi plate in the compound 

 plate, — all the plates, however low they may be, from crowding 

 and growth-pressure, being primaries. (Plate V. fig. 6 and 6 a.) 



The specimen of Echinothrix Desori, Agass., which I have used 

 in this research was large and well grown, and I have chosen one 

 of the plates, about the tenth from the peristome, which has only 



