226 ROBERT TRACY JACKSON ON ECHINI. 



to be the axis III, 5. This is the onlj' known case of a regular sea-urchin with centrally placed 

 periproct that is bilaterally symmetrical through an antero-posterior axis. The ambulacra 

 ventrally are twice as wide as dorsally and the ventral ambulacral plates develop a series of 

 short spurs which extend into the body of the animal. Somewhat comparable proximal growths 

 are known in Recent cidarids (p. 61; text-fig. 224; Plate 3, figs. 12, 13), but excepting Hyatt- 

 echinus are unknown in other Echini. There are eleven columns of plates in each inter- 

 ambulacrum in beecheri. The plates by their incoming show an accelerated development, 

 and dorsally the adradial columns and some others drop out, showing senescence. The only 

 known specimen is an internal mold so that the exterior of the plates is unknown, yet so perfect 

 is the specimen that it is represented spread out by the Loven method (Plate 26), (p. 291). 



Pholidechinus gen. nov. is known only from a single species, P. hrauni sp. nov. (Plates 

 27, 28). The test is high and spheroidal, ambulacra narrow throughout; at the mid-zone 

 alternate plates are • somewhat pinched off at the suture with the interambulacrum, by the 

 broadening of the plates between. Occasional!}' a plate is quite cut off from the interambula- 

 crum and thus becomes occluded, but this is exceptional. There are nine or ten columns of 

 plates in each, interambulacral area. The plates are scale-like and imbricate strongly. Spines 

 and tubercles are secondaries only, on both ambulacra and interambulacra. Genitals are 

 very wide with many genital pores. A very perfect lantern is known, as figured (p. 299). 



The family of the Palaeechiuidae (commonly known as the Melonitidae) contains the most 

 species, and as a whole is the best known of the Palaeozoic groups of Echini. The test is sphe- 

 roidal or occasionally elliptical and in one species obovate. There are from two to twelve 

 columns of plates in each ambulacral area according to the genus or species. It must be urged 

 that this character should be ascertained at or near the mid-zone, as ventral or dorsal to this 

 zone the number of plates may difTer (text-fig. 237, p. 231), and it is at this area that the full 

 differential specific characters are evinced, as elsewhere discussed (p. 55). In the interambul- 

 acra there are from three to eleven columns of plates in each area varying with the species, 

 also varying somewhat in the species and even somewhat in different areas of the same indi- 

 vidual. The plates of the corona are not imbricate, but in all species the ambulacral plates 

 bevel over the interambulacral on the adradial sutures (Plate 38, fig. 9). A reverse condition 

 exists in all Echini with imbricate plates, for in these the interambulacrals are beveled over the 

 ambulacrals on the adradial suture (text-fig. 32, p. 75). Coronal pfcites are very thick, more 

 so than in any other group of Echini, the thickness often exceeding the width or height of a 

 plate, a distinctly specialized character. The primordial interambulacral plates are resorbed, 

 and apparently these only, as there are two interambulacral plates in each interambulacral 

 area in the basicoronal row (text-fig. 25, p. 70). The peristome is known onl}^ in Melon- 

 echinus multi-porus (text-fig. 48, p. 80). On the basis of this species there are many rows of 

 ambulacral with some interradial non-ambulacral plates on the peristome. Ocular ]ilates are 



