ARCHAEOCIDARIS. 26,5 



to the right of the center, then in the adjustment of parts we find the arrangement of areas as 

 in A, I and G in text-fig. 239 bis. Or if column 4 originates to the left of the center, we then 

 have the arrangement as in areas C and E of text-fig. 239 bis. There are no gill slits (p. 223). 



Passing dorsally up to the mid-zone, the hexagonal plates progressively increase in height 

 relatively to their width and above the mid-zone this is still more marked. In the drawings of 

 the Munich specimen the basal terrace is not brought out. My attention was not called to 

 this feature when the drawings were made, but other specimens show it clearly on all plates 

 up to the mid-zone, as in Plate 11, fig. 4. Above the mid-zone, however, in younger, though 

 still large plates, the basal terrace is wanting. This is of much interest, as if such plates occurred 

 alone as fossils, one would assume that the species had no basal terrace, and this probably 

 accounts for the absence of a terrace as far as known in some species. Also, it is a condition 

 like that of Eocidaris which as a genus (most imperfectly known) has no basal terrace. Desor 

 referred A. rossica to Eocidaris, and he was probably misled by seeing only dorsal immature 

 plates in which the terrace had not yet developed. Still further dorsally, as we pass into the 

 area of really young plates, the scrobicule is first small and then wanting, the tubercle is imper- 

 fectly developed, then imperforate, and finally, in the younger plates, quite wanting, the plates 

 close to the apical area being smooth without tubercles of any kind. This progressive develop- 

 ment of characters of interambulacral plates as they grow older and are pushed ventrally, is 

 exactly the same as may be traced in the development of fossil or Recent cidarids (Plate 3, 

 figs. 1, 2) in some species of which it is especially marked (p. 106). In the A. rossica it is dis- 

 appointing in such a perfect specimen not to have the oculars and genitals defined. There are 

 many small plates in the center which are evidently periproctal; two of them lying above 

 area D, and larger than the rest, may possibly be oculars or genitals (Plate 11, fig. 3). 



Trautschold (1868) figured nearly complete lanterns in this species, but a specimen in the 

 Munich Museum and some fine lanterns that Dr. E. O. Hovey kindly procured for me in Russia, 

 now in the Museum of Comparative Zoology Collections, gave the opportunity to study details 

 and to show somewhat better figures than Trautschold's. These are seen in text-fig. 208 (p. 

 184), and Plate 12, figs. 1-8. The lantern is depressed, subtending an angle of about 90 degrees. 

 The pyramids are wide-angled, with curving sides which admitted of relatively long inter- 

 pyramidal muscles. The foramen magnum is moderately deep. The teeth in place extend 

 just above the base of the foramen magnum. The epiphyses are narrow, capping the half- 

 pyramids, and present a glenoid cavity for interlocking with the condyles of the brace. The 

 brace is block-shaped, as in Recent regular Echini. In one choice specimen (Plate 12, figs. 

 1, 2) a compass with bifid outer end is in place resting on the brace. This is the only good 

 compass known in the Palaeozoic, though I also show one in Pholidechinus brauni and Meeke- 

 chinus elegans. The sides of the pyramid are corrugated for the attachment of interpyramidal 

 muscles. This lantern, as earlier discussed (pp. 81, 82), is very much like the lantern of young 

 cidarids, also young Strongylocentrotus. 



