SCHIZASTER. 27 



and secondary tubercles. Within the fasciole the interambulacral spaces bear conspicuous 

 primary tubercles, scattered. These are very small, perforated, and placed on elevated 

 bosses surrounded by a broad excavated areola. Beneath, in the neighbourhood of and 

 behind the mouth, the primaries are more numerous, aud regularly arranged. The caudal 

 fasciole is distinctly seen. 



The length of a specimen in the Museum of Practical Geology is one inch and six 

 tenths, by rather more than an inch and a quarter in breadth. 



This fine addition to our British lists was discovered in the clays at Barton, during the 

 researches of the Geological Survey. I have dedicated it to the Marchioness of Hastings, 

 whose indefatigable researches among the Tertiaries of the cliffs at Hordwell and Barton 

 have contributed greatly to our knowledge of the organic remains of Eocene strata in Great 

 Britain. 



I have compared this species with excellent examples of Biaritz Eupatagi, kindly 

 communicated by Mr. Pratt; from all of them, however, it differs essentially. 



Genus Schizaster, Agassiz. 



Body cordate, depressed, broad ; apical disc placed far back ; ambulacra lodged in very 

 deep depressions, unequal, surrounded by a peripetal fasciole, from which a lateral supple- 

 mentary fasciole proceeds on each side towards the anus, and passes beneath it. 



1. Schizaster D'Urbani. {See Woodcut at the end of this Memoir.) 



Mr. D'Urban has communicated a sea urchin from the Barton beds in Alum Bay, appa- 

 rently belonging to this genus, but retaining so few fragments of the test that it is impos- 

 sible to pronounce upon its true position with certainty. 



Its outline was broadly cordate. As in many Schizaster 's, the postero -lateral ambulacra 

 are short compared with the anterolaterals. The former are oblong and contain about 

 eighteen pairs of pores in each series. The latter are broadly lanceolate and arcuated, and 

 have about twenty-seven pairs of pores, lodged in rather broad transverse grooves in each 

 series. Both antero- and postero-laterals are placed in deep depressions of the test. 

 Between the former and the odd or anterior ambulacrum, the test is swollen and pinched, 

 as is usual in this genus. The odd ambulacrum is long and broad, seated in a deep steep- 

 sided, flat-based sulcus. The other characters of the species are not sufficiently clear for 

 description. Fragments of two other specimens occur in the same slab with that here 

 described. It was brought to me too late for being included in the plate, and has conse- 

 quently been figured in a vignette at the close of this memoir. 



