SPATAGODESMA. 



199 



ciole, which has passed down to the actinal surface (PI. 108, fig. 6), while in 

 the younger specimen it runs close to the anal system (PI. 108, figs. 3, 4)- 

 In the larger Agassizia, however, important changes have taken place in the 

 ambulacral system. On the actinal side the phyllodes have become devel- 

 oped on from two to three pairs of actinal ambulacral plates (PI. 108, fig. G). 

 The odd anterior ambulacrum alone has retained its simple pores; the pos- 

 terior lateral ambulacral plates are, within the fasciole, perforated by two 

 pairs of pores, while in the anterior lateral ambulacra only the posterior zone 

 is thus perfoi-ated, the plates of the anterior zone retaining, as is charac- 

 teristic of this genus, the primitive single pore of the younger stages 

 (PI. 108, figs. 7, S, 9). The actinal system even in the younger stage is 



O lllllt. 



Fig. 280. Agassizia excentrica. 



It) lmn. 



Fig. 2S7. Agassizia excentrica. 



already transversely elliptical (PI. 108, fig. 2), slightly heart-shaped from 

 the development of the labium, and becomes still more so in the older 

 stage (PI. 108, fig. 6), where the labium is well marked. In both stages 

 the actinostome is covered with few large plates. 



It is interesting to note the gradual accumulation of secondaries and prima- 

 ries along the line (and below it) of the peripetalous fasciole (PI. 108, figs. 2,3, 

 6, 7, S). In the youngest Agassizia the central apical plate, with two indefi- 

 nite sutures indicating two genital and one interambulacral plate, is made up 

 of the ankylosed genital plates and shows no traces of genital openings or 

 of the madreporic body from the exterior (PI. 108, fig. .?), but from the 

 interior of the test, Fig. 286 (PI. 108, fig. 5) two well developed genital 

 openings are seen as well as the base of the stone canal and indications of 

 the sutures of the four genital plates. In the older stages there are four 

 genital openings, Fig. 287, piercing the central apical plate (PI. 108, fig. 7). 



