74 ROBERT TRACY JACKSON ON ECHINI. 



echinidae. On the other hand, ventrally in Clypeaster, the interambulacra bevel over the 

 ambulacra (text-fig. 234, p. 197) as in Pholidechinus or other types that have imbricate plates. 



A succession of bevels, in a meridional or transverse series, as Bather (1909a, p. 64) says, 

 produces imbrication properly so called. Imbrication is a character of many quite independent 

 series in Echini, and while of genetic value in a limited series, cannot be used as a basis of close 

 systematic connection in Echini as a whole, as I have shown (1896, p. 237). 



As imbrication is confusing, especially in fossils where specimens are often viewed from 

 the internal side, diagrammatic figures are given (text-figs. 32-39), illustrating the chief char- 

 acters. Imbrication in Echini is curiously constant in its direction. Interambulacral plates 

 in the corona always imbricate aborally in the vertical plane, so that the dorsal border of one 

 plate is inclined over the ventral border of the next adjacent dorsal plate of its series (text-fig. 

 32). Interambulacral plates also always imbricate outwardly or adradially from the center, 

 so that each plate from the center of the area laterally imbricates over its neighbor toward the 

 ambulacral area and on the adradial suture imbricates over the ambulacral plates, as seen in 

 Lepideslhes wortheni (Plate 67, fig. 8). 



In the ambulacrum, on the contrary, the imbrication is always adoral, so that the ventral 

 border of one plate inclines over the dorsal border of the next adjacent ventral plate, in the 

 opposite direction from that of the interambulacral plates (text-fig. 38). Laterally there is 

 little or no imbrication in ambulacral plates (Lepidesthes) except on the adradial suture, where 

 the ambulacral plates dip under the adjacent interambulacrals instead of over them, as they 

 do in the Palaeechinidae and in some clypeastroids, as above noted. 



The imbrication of ambulacral and interambulacral plates, as described, occurs weakly 

 in Archaeocidaris, Lepidocidaris, Lepidechinus, strongly in Lepidocentrus (Plate 20, fig. 8), 

 Hyattechinus (Plate 22), Pholidechinus (Plate 28, fig. 2), Perischodomus (Plate 64, figs. 2, 3), 

 Lepidesthes (Plates 66, 70), Pholidocidaris (Plate 73, fig. 4), and Meekechinus (Plate 76, fig. 1). 



Imbrication also occurs in the modern thin-plated Echinothuriidae. This character has 

 been considered a basis of connecting the Echinothuriidae with Palaeozoic types having imbri- 

 cate plates, but so many characters are opposed to it, that this must be considered a case of 

 parallelism. In modern forms, imbrication is not limited to the Echinothuriidae, but occurs 

 also in the Centrechinidae. This character is well developed in Astropyga pulvinata (text-fig. 

 100, p. 109), where interambulacral plates imbricate dorsally and laterally, and the ambulacral 

 ventrally. The plates are I5o thin that it is difficult to see the sutures of imbrication in cross 

 section, but the imbrication is readily seen on the internal view of the test. The interambula- 

 cral plates imbricate over the ambulacrals, as they do also in echinothuriids, in Centrechinus, 

 and in the Palaeozoic types with imbricate plates (as noted in the Palaeechinidae, the ambula- 

 cral plates are beveled over the interambulacrals, but imbrication proper does not occur in 

 this family). Mr. Agassiz (1881, p. 71) notes the occurrence of imbrication in Astropyga, but 



