86 ROBERT TRACY JACKSON ON ECHINI. 



it seems, as shown by Loven (1892, p. 18) and A. Agassiz (1904, pp. 31, 96, 97, et al.), that they 

 are derived from the corona. As Loven describes it, the ambulacrals flow downward between 

 the interambulacrals and pass as if discharged through the outlet of a river on to the buccal 

 membrane. Such is the condition in the Palaeozoic Hyattechinus (Plate 23, fig. 1), Pholi- 

 dechinus (Plate 28, fig. 1), Melonechinus (Plate 56, fig. 8), modern Eucidaris (text-fig. 46) and 

 Phormosoma (text-fig. 43). 



Loven (1892, pp. 22, 31) also assumed that the interradial plates on the peristome of Cidaris 

 and the granules and small non-ambulacral plates of " Ectobranchiates " are formed by the 

 disintegration and remodeling of coronal into buccal plates. He speaks of coronal plates as 

 disintegrating and the calcareous substance being redeposited as buccal plates. This seems 

 a quite untenable view. If the plates are dissolved, that should be the end of them, and we 

 cannot assume that buccal plates are derived from the resulting lime any more than the thicken- 

 ing of teeth or any part of the test. I believe no one has shown the interambulacral plates 

 separating from the corona in Echini and passing on to the peristome, there to be represented 

 by actually transferred larger or smaller plates. Outside of the transferred ambulacral plates, 

 as seen in Cidaris and similar types, it seems that all plates of the peristome originated in that 

 area and cannot be considered as derived from the corona. In spatangoids the peristome is 

 usually densely plated, yet no resorption of the base of the corona has occurred, and the pri- 

 mordial ambulacral and interambulacral plates are in place in the basicoronal row, therefore 

 no transfer of coronal ijlates can have taken place. 



Ocular and Genital Plates. 



The range of characters presented by the ocular and genital plates is most interesting. 

 Their mutual relations are discussed here, and in the next section certain considerations of the 

 genital plates by themselves are taken up. Cases of marked aberration occur in some spatan- 

 goids as CoUyrites, and especially Pourtalesia; these I have not studied, and they are eliminated 

 from the discussion. The characters of oculars and genitals are typical of the several groups 

 of Echini, and the later forms present striking relations to Palaeozoic genera. An ocular plate 

 in Echini overlies an ambulacrum wholly and the two adjacent interambulacra in part on either 

 side. Immediately on the ventral borders of the oculars all coronal plates originate. It 

 seems that at this point the tissues exist which give rise to new plates. As earlier discussed 

 (p. 62), the corona may be considered as made up of five areas, each of which is directly con- 

 nected with one of the ocular plates, the areas being each an ambulacrum and two half- 

 interambulacra (text-figs. 217, 218). The five oculars are always present barring the 

 excepted Pourtalesia. The genitals overlie the interambulacra in part, but not the lateral 

 borders of the same, and never reach the ambulacra. In some cases the genitals may not 



