ECHINOIDEA. 27 



Eemarks. — The genus Eemipneustes was founded by Agassiz in 1835 on two 

 remarkable urchins, one of which was the well known Spatangus radiatus. Lam., 

 from Maestricht ; the second was M. africanus from the upper cretaceous beds 

 of Algiers. Agassiz characterises the new genus as follows : — 



" Test 61ev^, trfes epais. Un sillon ant^rieur profond. Ambulaeres pairs a fleur 

 de test, zones porif feres in6gale8 ; les exterieures ayant des pores allonges transversale- 

 ment, les int&ieurs de simples trous ronds. Anus au bord post^rieur. Quatre 

 pores gdnitaux s6pares par une plaque interm^diare. Point de trace de fascioles, ni 

 p6ripetales, ni sous anal. Deux espfeces de la formation cr6tac^e." 



The above diagnosis has been slightly amended by D^sor,' and I fully agree 

 with that author that a genus thus characterised should be easily recognised. Not- 

 withstanding this and although fully realising the remarkable feature of lateral am- 

 bulacra, forbes^ is of opinion that the genus Semipneustes is identical with 

 the genus Toosaster, but he apparently forgets, as D^sor justly remarks, that the 

 ambulacra of Memipneustea are open and not petaloid, and that the apical disc is 

 elongated. 



With more apparent justification, d'Orbigny' identifies Semipneustes with Eol- 

 aster, dwelling in detail on the characters which are common to both genera, but, 

 as Ddsor remarks, " la profondeur du sillon anterieur, le contraste frappant entre 

 les deux zones porifferes d'un meme ambulacre " are characters which are quite 

 sufficient to justify the independence of the genus Semipneustes. 



D6sor's view has been generally accepted, and I think that if the characters as 

 given in the above description of the genus are accepted, the genus Semipneustes 

 forms so characteristic a group that even in badly preserved specimens its identity 

 cannot be mistaken. 



Up to the present date five species of Semipneustes were known, to which 

 must be added a new one, so that there are now six species altogether, viz., — 



1778. Hemipneustes striato-radiatus, Leske. 



Although the shape of the test varies considerably among these six species 

 they all exhibit the generic characters in such a marked and uniform way that in my 

 opinion it is only the shape of the test to which we have to look for specific distinc- 

 tions. For this purpose the following two tables will give some interesting informa- 

 tion. The first table contains the ratio b-^L (breadth divided by length) and the 

 second the ratio h-^L (height divided by length). 



h-^L. 

 M, leymeriei ....••= 0'95 — '98 

 II. compressus ,.....= 0*91 — '92. 



^ Synopsis des Echinides fossiles, 1858, page 349. 



" Geological Survey of the United Kingdom. British Organic remains, Decade IV. 



fig. 1, 1852. 

 ^ Palloutologie frangaise : Terr, cr^t. Echinoides ineg., page 72. 



E 2 



