174 ROBERT TRACY JACKSON ON ECHINI. 



Eucidari.f tribtiloides in structure (text-figs. 62-69, p. 98). In the Lepidesthidac, the peri- 

 proctal plat os in the known types are essentially the same, like cidarids, but are rather thinner 

 and more scale-like; such are shown in Lepidesthes formosa (Plate 68, fig. 5), L. collelti (Plate 

 71, fig. 1), and Meekechinus elegans (Plate 76, fig. 6). Palaeozoic types as far as known do not 

 throw any light on the suranal plate, which is such a prominent feature in the young of some 

 Recent Echini and the adults of certain genera. There is no evidence that a suranal plate 

 was differentiated in the Palaeozoic, and, as it is apparently wanting in the Cidaroida and 

 Aulodonta, both young and adult, it is also doubtless wanting in the Palaeozoic. 



In the Cidaroida, periproctal plates are thick and angular, filling the area, with the anus 

 central. They are essentially like those known in the Palaeozoic and in so far may be con- 

 sidered primitive. These plates are shown in representative cidarids in text-figs. 59-74, 164 

 (pp. 94-99, 149). Love'n (1892) figures a young Goniocidaris with a single plate filling the 

 periproct (Plate 2, fig. 3); but Mortensen (1911) thinks this a mistake, as in a specimen of a 

 similar stage he found three plates. Mr. Agassiz's (1904) figures of young cidarids, and those 

 young specimens which I have studied, show no indication of a dominance of a first, over later 

 added plates. It seems, then, that we have here no suranal such as exists in the Saleniidae 

 and in young Echinidae and Strongylocentrotidae, in which groups alone has it been demon- 

 strated. The existence of a suranal plate is apparently a secondary and specialized rather 

 than a primitive character. Dr. Mortensen wrote me in effect that he concurs in this view, 

 and he expresses the same in a recently published paper (1911). 



In the Centrechinoida the character of periproctal plates presents wide differences in various 

 genera. Of the Aulodonta there are large plates on the periphery of the periproct with small 

 Isolated plates within, as seen in Chaetodiadema, Centrostephanus, Astropyga (text-figs. 

 96-99, pp. 108, 109), or young Centrechinus (text-fig. ^88) ; or there may be small, isolated 

 plates only, with tissue largely leathery, as in adult Centrechinus (text-figs. 93-95, p. 107). Of 

 the Stirodonta in Salenia and Salenocidaris (Plate 4, figs. 1, 6; text-fig. 102, p. Ill) we find 

 a prominent suranal plate lying dorsal to genital 3 and with small anal plates in addition. In 

 the fossil Peltastes (Plate 4, fig. 7) the suranal occupies a prominent place but lies dorsal to 

 ocular III, thus differing from Salenia and young Strongylocentrotus and Echinus, in which 

 the suranal lies dorsal to genital 3. In a series of 74 specimens of Peltastes wrighti one speci- 

 men has two large plates, as in Aerosalenia hemicidaroides (text-fig. 104, p. Ill), and as in 

 that figure the larger plate, which may be considered the suranal, lies against genital 3, as 

 in Salenia. This is a striking difference from the other specimens, in which there is only a 

 single plate lying dorsal to ocular III, as shown in Plate 4, fig. 7, which is the generic character. 



The fact that the suranal may occupy one of two positions has a certain bearing on other 

 genera. In Acrosalenia spinosa (text-fig. 103, p. Ill) the suranal lies dorsal to ocular III, as 

 in Peltastes, but in A. hemicidaroides (text-fig. 104, p. lit) there are two large plates; the larger 



