BRITISH FOSSILS. 



Decade IV. Plate V. 



HEMIPNEUSTES GHEENOVII. 

 [HEMIPNEUSTES. Agassiz. (Sub-kingdorii Radiata. Class Ecliinodermata. Order 

 Echinoidea. Family Spatangidte.) Body cordate, more or less tumid, in some species 

 much elevated ; ambulacra dissimilar, the lateral ones dorsally curved, petaloid or semi- 

 petaloid, the odd one straight and lodged in a sulcus. Vent terminal supra-marginal. 

 Mouth excentric, transverse. No fascioles. Genital pores four. Tubsrcles minute, per- 

 forated, placed on crenulated bosses.] 



Hemipneustes Greenovii. Sp. Nov. 



Diagnosis. H. testa tumidd amhitu cordato ; amhulacro antico poris 

 superne dissimilarihus alternatis ; amhidacris antero-lateralibus serie ex- 

 terno pororum angustissimo, posterO'latercdihus seriebiis sub -similar ibus. 



This curious and rare Ecliinite presents cliaracters which distin- 

 guish it very strikingly from any described species. Its characters 

 link together the genera Hemipneustes and Toxaster, hitherto 

 placed apart, even in different groups, the former being usually 

 associated with Ananchytes, the latter with Micraster. 



The well known Hemipneusies radlatus of the Maestricht 

 Chalk is the type of its genus. In general contour it resembles an 

 Ananchytes, but essentially differs in the structure of the test, for 

 instead of the ambulacra beinj^ all similar and formed of homo- 

 geneous or nearly homogeneous parts throughout, the lateral ones 

 are very unlike the anterior one, the former being partially petaloid, 

 and the latter rectilinear. Its lateral ambulacra are, moreover, 

 remarkable for being composed in their petaloid portions of 

 differently constructed avenues, the hinder avenue of each being 

 made up of pores widely sepa^rated and connected by a broad groove, 

 whilst the fore one is made up of minute and closely approximated 

 pairs of pores. 



The genus Toxaster, on the other hand, was instituted for tliose 

 heart-shaped urchins that resemble Holaster in general aspect, but 

 have subpetaloid ambulacra in the manner of Micraster, differing, 

 however, materially ^from the latter in the absence of any kind of 

 fasciole. The depressed or slightly tumid shape of most of the 

 known species is so very unlike that of Hemipneustes radiatus, 

 that no comparison seems to have been instituted Avith it. Essen- 



[IV. v.] 4 F 



