GEKEBA AND GEOUPS OF THE ECHINOIDEA. 39 



is the imperfect calcification of plate areas; the plates are 

 therefore surrounded by soft tissue, and that of one plate is con- 

 tinuous with those of its neighbours, without close suturing. 



It must be admitted that the overlap in Astropyga is rather an 

 overstrained analogue of that which occurs in Asthenosoma ; it is 

 slight and persistent in Astropyga, and there is no movable soft 

 tissue between the plates. 



The interesting work of the Drs. Sarasin upon Asthenosoma 

 shows that the longitudinal internal muscles have to do with 

 the movements of the plates. The same authors discovered 

 five large tubular branchiae (Stewart's organs), passing out below 

 the " compasses " and occupying much vertical space in the am- 

 bulacral areas, so that an exaggerated condition of what was 

 formerly considered peculiar to the Cidaridse prevails. The outer 

 branchiae were well drawn by A. Agassiz. The jaws and teeth 

 resemble those of Cidaris, but the perignathic girdle has ambu- 

 lacral processes, besides ridges upon the interradia. The want of 

 interradial peristomial membrane plates is another important 

 character which separates the genera from Cidaris ; and the 

 ambulacral plates differ entirely in their method of construction 

 from those of that genus, and approach those of the Diadematidae 

 very slightly. The construction of the apical system is almost 

 anomalous. 



It is certainly not correct to state that the alliance of the Echi- 

 nothuridae is close with the Palseechinoidea, for there is but 

 slight resemblance between the thick-plated forms of Archaeo- 

 cidaridae and the group now under consideration. Even those 

 genera of the Perischoechinoidea with bevelled plates which 

 are all thicker than those of the Echinothuridae, show no evidence 

 of defective calcification nor of the presence of interplate, movable 

 membrane, and the structural characters of Asthenosoma, for 

 instance, are scarcely recognized. A. Agassiz has shown that 

 considerable bevelling occurs in some thick-plated spherical 

 Mesozoic and recent Echinoidea, and it does not appear to have 

 been accompanied by movement and flexibility. Pelanechinus of 

 the Oolites is the earliest known Echinothurid, and it is more 

 Diadematoid than its successors. 



During the Oolitic age there was evidently much variability 

 in the structures of the ambulacra of the original Diadematidae, 

 and the entrance of the radial plates within the periproctal ring 

 was not uncommon. It appears more reasonable to place the 



