2 



BKITISH FOSSILS. 



tially, however, there appears to be no generic distinction between 

 them, for though in several Toxasters the difference between the 

 two series of avenue pores in the lateral ambulacra is very slight 

 and indistinct, in others it is as great as in Hemipneustes proper. 

 In Toxaster complanatus for example the two avenues of each 

 lateral ambulacrum are similar ; so are they also nearly but not 

 quite in Toxaster gihbus (Agassiz), T. Ricordeanus (Cotteau), and 

 T. Nicceensis (Sismonda). But in Toxaster ohlongus (Deluc) and 

 T. Verany (Sismonda) they are nearly as dissimilar as they are in 

 Hemipneustes radiatus ; and in the species now to be described the 

 antero-lateral ambulacra have dissimilar avenues, and the postero- 

 laterals nearly similar ones. Unless the Toxaster ohlongus, 

 T. Moulini, T. semistriatus, and T. Verany be regarded as 

 species of Hemipneustes, and the name Toxaster be confined to 

 those species resembling T. complanatus in the arrangement of the 

 pores, (in which case my new species would become the type of a 

 new and intermediate genus,) these two groups must be considered 

 as forming but one natural genus. Such indeed is the view that I 

 take of the assemblage, and the name Hemipneustes, as the older 

 appellation, had best be retained for it. 



The Hemipneustes Greenovii has a regularly cordate outline 

 truncated posteriorly. It varies greatly in degree of tumidity, but 

 appears always to present a high apex and hinder half, whilst the 

 portion in front of the centre declines with a gentle curve, and is 

 divided into two tumid cheeks by the rather deep and wide but 

 not abruptly margined central furrow. The hinder extremity is 

 perpendicularly truncate, and bears the vent at about half the height 

 of the back. The under surface is flattened, but very slightly 

 convex, and rounded at the side. The mouth, which is transversely 

 oblong, is placed in a depression very near the anterior margin, 

 opposite to the anteal furrow. 



The ambulacra require to be separately described. The odd one 

 is one third wider than the laterals. It preserves nearly an equal 

 width throughout, and is composed of plates which become con- 

 tracted vertically throughout its upper half Out of about 37 plates 

 seen in each of its two series, the lower seven are~'square or oblong, 

 and large. They are each perforated near their outer and lower 

 corners by a pair of minute approximated pores. In the narrow 

 plates above these a very curious change takes place in the avenue 

 arrangements for the pairs of pores ; all become conspicuous, are 

 alternately approximated, and set widely apart ; the latter pairs 

 each consisting of a long outer pore and a shortlnner one. Inside 



