SALENIA. 259 



in which case the anal system appears pushed hack posteriori}' as in Salenia, 

 owing to the original spiral condition of the Sea-urchin, which continues to 

 affect the growth during the whole time of its existence (see figs, of young 

 Cidaris) ; the abactinal system sometimes shows very plainly the spiral mode 

 of growth by not being perfectly symmetrical, that is, the plates on the two 

 sides of the longitudinal axis are not equal in size ; this is particularly the case 

 in some species of Salenia. The structure of the abactinal system of our liv- 

 ing species. Salenia varispina, fully corroborates the view taken of the proper 

 homology of the subanal plate of the Salenidae. The analysis of the abactinal 

 system of a living Salenia here described shows that the subanal plate is the 

 homologue of the first-formed anal plate of young Echini (which iu many 

 cases remains decidedly larger even in older stages, Salmacis, Temnechinus, 

 Trigonociclaris), and shows that the abactinal system of Salenia is entirely 

 homologous with the abactinal system of the other Echinoids, the original, 

 first-formed plate only retaining a greater preponderance than has been thus 

 far noticed in other genera. Add now to this eccentric position of the anal 

 system the presence of a large plate covering (as in young Echini) nearly the 

 whole of the anal system, and we have then the remainder of the anal system 

 covered by excessively small plates (as in young Echini), lost, of course, in 

 the fossil Salenidae thus far found. This shows that the subanal plates have 

 no special function, are not special plates found in the group of Salenidae 

 alone, but are simply an embryonic feature, retained in the adult, — as is the 

 case in other genera, where, instead of one plate, we may have several 

 (Arbacia, etc.), — so that this subanal plate is simply a part of the plates cov- 

 ering the anal system, and has nothing to do in the apical system with the 

 genital and ocular plates ; although from its size it appears (as in young 

 Echini) to be a most prominent feature, and to form part of the genital 

 plates in the abactinal system. This so-called subanal plate is readily recog- 

 nized as such in Temnechinus (see PI, VIII. f. 8), while even iu genera in 

 which the existence of the subanal plate has never been claimed (Temno- 

 pleurus, Salmacis), we find that in the oldest specimens of some of the species, 

 this first plate, Avhich at first covered the whole anal system, always retains 

 a marked predominance, and can readily be distinguished from the others, 

 subsequently added to cover the increasing size of the anal system. 



The remaining part of the anal system was, in the fossil species, undoubtr 

 edly covered by small plates, as in the living species ; and that this was the 

 probable structure of the anal system is shown by Wright, who has figured 



