12G Prof. P. M. Duncan and Mr. W. P. Sladen on 



It has been stated by more than one author that when the 

 apical disk of Acrostxlema hemicidaroides is absent the speci- 

 mens cannot be distinguished from those of a Hemicidaris 

 deprived of that system. 



But there are several important structural distinctions between 

 the species of the two genera. For instance, the branchial 

 cuts of the Acrosalenia are large and those of Hemicidaris are 

 small; the great tubercles oi Hemicidaris coyer three plates 

 arranged in a compound geometrical plate, and this is never 

 the case in the Acrosalenia^ which has the tubercle followed 

 by a large granule in vertical succession, the tubercle being 

 in relation with two plates of a compound one and the granule 

 being upon a small, low, separate primary. The construction 

 of the ambulacral plates differs completely in the two genera, 

 and if the special arrangement of the great compound plates of 

 a Hemicidaris described in Quart. Journ. Geol. Soc. vol. xli. 

 p. 438, figs. 13, 14,15, 16, is compared with similarly placed 

 ones of A. hemicidaroides (fig. 5), the distinction becomes 

 obvious ; there are three component plates in the one and but 

 two in the other species. There are no instances of compound 

 plates made up of four plates in A. hemicidaroides ix?> in Hemi- 

 cidaris, and the slope of the plates and the directions of the 

 sutures differ in the two forms. Near the peristome of A. 

 hemicidaroides the crowding produces triplets, and they are 

 remarkable, for, unlike the arrangement in A. spinosa, the 

 adoral primary is long and low, tiie median primary is large 

 at the median line, and the aboral primary is almost a derai- 

 plate. The adoral suture of the last-named plate is much 

 curved. There is a certain amount of resemblance in the 

 peripodia of the two sets of forms, but those of the Acrosalenia 

 (figs. 6 a and /3) are characterized by a very prominent inter- 

 porous knob which stands up well beyond the level of the 

 test. 



Acrosalenia jJUstuJata, Forbes, was carefully studied by 

 Wright {op. cit. p. 242, pi. xvi. figs. 2 a-^), who remarks 

 upon its variability in specimens of the same dimensions, and 

 of course in those of difierent ages. But Wright's speci- 

 mens do not appear to have enabled him to give a figure of the 

 apical disk which could be correct, and his descriptions of the 

 structure are, naturally, not positive. His studies regarding 

 the variability of the tests were directed to explain Forbes's 

 seeming inaccuracy about the separate condition of the scrobi- 

 cules of the large interradial tubercles and about the median 

 area only having two rows of tubercles. Wright showed 

 that Forbes's specimen was immature. In consequence of 

 this perfectly correct view, Wright gave a new diagnosis of 



