HOLASTER. 309 



Gemis — HoLASTER, Agassiz, 1839. 

 Synonym — Spatangus, Anct. 



The test has an oval or corcliform figure, more or less convex at the upper surface, 

 and, in general, flat at the under surface. 



The pairs of antero- and postero-lateral ambulacral areas are apetaloidal, superficial, 

 and lanceolate ; they are widely apart below, and join each other above. 



The poriferous zones are formed of elongated pores (always those in the external 

 rows), disposed by simple pairs, very much separated and regularly superposed on each 

 other, between the ambitus and apical disc. The ambulacrum, or single anterior 

 area, is always lodged in a shallow central sulcus ; its pores are very minute and 

 resemble those of the lateral areas. The ambulacral summit is central or subcentral. 



The apical disc is solid and elongated ; its elements consist of four perforated ovarial 

 plates and five perforated oculars. The genital plates arc disposed in pairs, and the 

 anterior are separated from the posterior pair by a pair of ocular plates being interposed 

 between them. 



The posterior border is narrow and more or less truncated, and the periprocte, 

 always marginal, opens in general in the upper part of this region. The mouth-opening 

 is situated near the anterior border in a slight depression, and the peristome is trans- 

 versely oval and imperfectly bilabiate. There are no fascioles on any part of the test. 



The shell is very thin, and the surface of the plates is uniformly covered with a very 

 fine granulation ; tubercles, more or less developed, appear in irregular order among 

 them. Granules attaining their largest size, in the form of tubercles, appear upon the 

 plates of the inter-ambulacral areas, and on those of the basal region, and this is more 

 especially so in Holaster Trecensis, Leym. The ambulacral areas have, however, fewer 

 tubercles than the other areas. The Holasters appertain to the Cretaceous Rocks, and 

 form leading fossils in certain stages of the same ; they are most abundant in the Upper 

 Greensand and Lower Clialk, one species only being special to the Wliite Chalk. 



There seems to have been an unnecessary amount of confusion in the synonymy of 

 some of the species, arising it would appear more from the copying of names than 

 from the inspection and comparison of the specimens themselves. 



In the British Islands we find : Holaster lavis, De Luc, in the Upper Greensand 

 and Lower Chalk ; Holaster obliquus, Wright, in the Upper Greensand and Lower 

 Chalk ; Holaster suhorhioilaris, Dcfrance, in the Chloritic Marl and Lower Chalk ; 

 Holaster Icevls, var. planus, Mant., in the Upper Greensand, Lower Chalk, and lower 

 portion of Upper Chalk ; Holaster subglobosm, Leskc, in the Chloritic Marl and Lower 

 Chalk ; Holaster Trecensis, Leym., in the Lower Chalk ; Holaster pillula, Lamk., in the 

 Upper Chalk. 



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