LEPIDESTHES. 413 



the center outward and over the ambulacrals on the adradial sutures. Those plates are strongly 

 rounded in outline, and on the exterior bear numerous secondary tubercles with corresponding 

 spines (Plate 67, figs. 8, 13, 14). There are no primary tubercles or primary spines known 

 in the genus. The interambulacral plates as seen from the interior imbricate adorally and from 

 the sides toward the center; and laterally bevel under the ambulacrals, just the reverse of the 

 exterior. On the interior also the plates are much less rounded and more sharply angular 

 than on the exterior, all as shown diagrammatically in text-figs. 32 to 38, p. 75. The primordial 

 interambulacral plates are in place in the basicoronal row (Plate 68, fig. 3), and, as far as known, 

 there are two plates in the second row, three in the third, and four in the fourth as usual in 

 the Perischoechinoida. In later growth a fifth and a sixth column may develop dorsally, as in 

 Plate 66, fig. 10, or one of the four ventral columns may drop out so that for most f the area 

 there are only three columns, as in Plate 67, fig. 8. 



The peristome is covered with ambulacral plates only (Plate 68, fig. 3). The apical disc 

 is small proportionately to the diameter of the test, and is rather fully known, oculars are 

 insert, separating the genitals, and with two pores each (Plate 68, fig. 5), which latter is a 

 very unusual character (p. 81) and perhaps represents an individual variant. Or the oculars 

 may be apparently quite imperforate (text-fig. 251, p. 428). The genitals are low and wide, 

 with two or three pores each. In some cases no pores are visible, but this is ascribed to pre- 

 servation and not to a real absence of pores. A madreporite with typical fine pores occurs 

 in two known cases (Plate 68, fig. 5; Plate 71, fig. 1). The periproct is plated with many 

 small, somewhat rounded plates, which are less angular than in the Palaeechinidae (Plate 68, 

 fig. 5; text-fig. 251, p. 428). The lantern is inclined, teeth are grooved, pyramids wide-angled, 

 with a moderately deep foramen magnum, and laterally with ridges for the attachment of 

 interpyramidal muscles. Epiphyses are narrow, capping the half-pyramids. The brace is 

 of the character typical of all regular Echini (Plate 68, figs. 9-14). 



Mr. Agassiz (1881, p. 79) in discussing the Echinothuriidae, says of the actinal system 

 (peristome) that "from the similarity of its structure in such genera as Archaeocidaris, Pholi- 

 docidaris, and Lepidesthes, we can fairly assume that the abactinal system [oculo-genital plates] 

 corresponds in its general features with that of the Echinothuridae." I think that this statement 

 is hardly justifiable. In the Perischoechinoida there is no evidence of gill cuts for peristomal 

 gills (p. 223), whereas these are present in the Echinothuriidae. 1 In Archaeocidaris there 



1 Mr. Agassiz (1874, p. 645) says, "It is well known that the Cidaridae and Echinothuriae have no cuts in tin- test 

 for the passage of the actinal gills, but we find these cuts directly at the point of contact of the buccal membrane and 

 the teeth, and there the gills make their appearance." (See also A. Agassiz, 1874, pp. 694, 699; 1881, p. 53.) The Ci- 

 daridae have no external or peristomal gills, but do have internal radially situated Stewart's organs (pp. 61, 62, 212). On 

 the other hand the Echinothuriidae do have small peristomal gills, which, as usual in the Centrechinoida (pp. 62, 213), 

 impinge upon the basicoronal plates of the interambulacra thus making slight reentrant cuts (text-figs. 43, 44; pp. 80, 

 83). The Echinothuriidae in addition to peristomal gills have internal radially situated Stewart's organs (p. 214). 



