c \i. Whit l;l\i - DIOMED i 79 



we have an incomplete series of rings of anal plates, three larger plates 

 with angular sides and fully as solid as the genitals and oculars, en. 

 closing in the arc they form a Bel of plates gradually diminishing in 

 si/.c to the outer part of the anal ring, which was probably filled by still 

 Bmaller plates. 



All the plates of the anal system of Aorosalenia being thus irregularly 

 arranged, it requires but a slight transition from this irregular arrange- 

 ment of the smaller anal plate- to a regular plating of a circular or ellip- 

 tical anal opening Hanked on two or three sides by three <>r four larger 

 and more compact plates, to pass into the structural condition of the 

 Salenidse, where, owing to the atrophy of some of the large anal plates, 

 the anal opening is flanked by only one large plate. This alone is closely 

 joined to the genital ring, and seems to form a part of it, rather than of 

 the anal system. In Acrosalenia we find a larger number of such plates, 

 passing so gradually from the larger polygonal anal plates to the smaller 

 irregularly shaped ones adjoining the anal opening, that there appears to 

 be no question to which system of plates they all belong. That such a 

 transition probably took place seems possible, if we compare the apical 

 system of Salenia varispina, with its eight small anal plates (Plate XXX. 

 Fig. 3) and its one large suranal plate, with the figures of the apical sys- 

 tem of Salenia Pattersoni (Figs. 1, 2). 



We may sometimes still detect a more or less indistinct trace of these 

 original fiye radial plates by the presence of five larger plates in the outer 

 anal ring, as in Toxopneustes variegatus (Plate XXXI. Figs. 8-10), in 

 Trigonocidaris monolini (Fig. 5). Prionechinus sagittiger (Chall. Echini, 

 Plate A*I. a Fig. 13), or in Cottaldia forbesiana (Ibid., Fig. 10). 



So that while we are undoubtedly justified in considering the single 

 anal plate of the young of such Echini as Strongylocentrotus (Plate XXXI. 

 Fig. 7), Trigonocidaris albida (Fig. 4), and Teinnechinus (Fig. 3) as homol- 

 ogous with the single large anal plate of the Salenidse, we are no longer 

 justified in looking upon this kind of anal system with a single plate as 

 the earliest type.* On the contrary, from the analysis that has preceded we 



* There is no Echiuoid known, as the Sarasins seem to suggest, in which in the adult a single plate 

 covers the anal system. Such a condition exists only in the youngest stages, and additional plates are 

 intercalated between it and the genital ring, the first plate to appear remaining the most prominent 

 during a part of the growth of the Urchin, while it always remains the most prominent in Salenia, and 

 was already so in its youngest stages. To suppose, as the Sarasins do, that the large suranal plate 

 m\y possibly be formed from the coalescing of a number of smaller anal plates, runs contrary to all 



