76 ROBERT TRACY JACKSON ON ECHINI. 



says the plates of both areas imbricate in the same direction dorsally. I have studied imbri- 

 cation in only one specimen, an Astropyga pulvinata; but in that one the ambulacra imbricate 

 ventrally, interambulacra dorsally as described. 



I believe the fact has been overlooked, but a weak imbrication occurs in Centrechinus 

 setosus. It is not obvious in external view, but it is readily seen on viewing the interior of a test. 

 The interambulacral plates imbricate dorsally as usual, and the ambulacral slightly ventrally; 

 the interambulacra are inclined over the ambulacra on the adradial suture as usual in imbricate 

 Echini, living or fossil. If a specimen of Centrechinus from alcohol is sectioned horizontally 

 by cutting through the ambitus, it is found that there is much flexibility of the test. The 

 same is seen in a dried specimen if it is first soaked in water. In Chaetodiadema the test is 

 thin and highly flexible. It is very interesting that flexibility and imbrication are so charac- 

 teristic of the Recent Centrechinidae, for many characters connect this family with the 

 Echinothuriidae, as clearly shown by Messrs. A. Agassiz and Clark (1909). The rule of ventral 

 imbrication for ambulacral and dorsal for interambulacral plates in the corona, is apparently 

 without exception in the corona for all Echini where imbrication occurs. Certain exceptions 

 according to the literature are said to exist. Worthen and Miller (1883) supposed that in 

 Lepidesthcs ( = Hybochinus) spectabilis the interambulacral plates imbricate ventrally and the 

 ambulacral dorsally. Their specimen was doubtless incorrectly oriented on account of the 

 position of the teeth as discussed under the systematic description of that species, so that if 

 I am correct it is no exception to the rule. 



In my earlier paper (Jackson, 1896, pp. 223, 224) a serioys error was made in regard to 

 Lepidocentrus mulleri where the interambulacra were described as imbricating ventrally; this 

 was wrong, since the specimen was inverted, as discussed under that species in this paper. 

 Again, an error was made in describing Pholidocidaris meeki (1896, p. 211, Plate 9, fig. 54). 

 The interambulacra and ambulacra were described as both imbricating adorally. The speci- 

 men (Plate 73, figs. 6, 7) is imperfectly preserved, and I can only plead guilty of an error of 

 observation. I was also partly misled by the fact that Meek and Worthen's (1873, Plate 15, 

 fig. 9a) figure of Pholidocidaris irregularis, the only good figure published, was inverted, a fact 

 that was since ascertained. There is no proof at present that imbrication of coronal plates is 

 ever other than dorsal in the interambulacra and ventral in the ambulacra in types of Echini 

 with imbricate plates. 



On the peristome, imbrication is adoral when it exists in both ambulacral antl interambula- 

 cral plates, as in Eucidaris (text-fig. 39, also text-figs. 43, 46, 47, p. 80). 



When imbricate specimens of fossil Echini are viewed from within (text-fig. 34), the struc- 

 ture appears very different from that as seen from without (text-fig. 32). This is shown well 

 in Perischodomus biserialis (Plate 62, figs. 6, 7; Plate 64, figs. 2, 3). From the interior, the 

 ambulacra appear to imbricate dorsally and laterally overlap the adjacent interambulacra; 



