6 



BRITISH FOSSILS. 



polygonal, so that the pores are more distant than they are on the 

 sides and margin. The other ambulacra, in consequence of the 

 exceedingly excentric position of the mouth, near which they widen 

 out, are much shortened and close set below, and the pores of all 

 five in the immediate neighbourhood of the mouth form a sort of 

 irregular star, the longest rays of which are formed by the buccal 

 extremities of the antero-lateral ambulacra. The mouth is lodo'ed 



o 



in an oblique depression ; its lips are thickened ; the hinder one is 

 highest, but does not overlap. The vent is set obliquely and sub- 

 marginally. 



The apical disl^ is much elongated. It is composed of four large 

 and perforated genital plates (the fifth is usually obsolete) and five 

 large oculars ; of the two anterior genital plfites, the right one is the 

 largest, in consequence of its including the madreporiform body in its 

 constitution. This organ is not conspicuous, but under the lens 

 its punctations, which are exceedingly minute, may be detected. 

 Of the two hinder genital plates, the left one is largest. They are 

 all polygonal. Of the ocular plates, the two antero-laterals are 

 much larger than the other three plates. The eye perforations 

 are marginal ; the genital holes sub-central. All the plates are 

 thickly covered with granules and bear a few tubercles. On flint 

 casts the position of the central portion of the apical desk is marked 

 by a space included within an oblong groove. This groove is 

 caused by the impressions of the walls of a solid oblique tube-like 

 body projecting from the inner surface of the apicpJ disk into the 

 cavity of tlie test, and directed backwards. This organ appears to 

 me to be the sand canal, and is connected with the madreporiform 

 plate. On flint casts the pores of the avenues are seen strongly 

 projecting, and on each side of the vent is a shallow groove mark- 

 ing the place of internal strengthening buttresses. 



The following table exhibits the comparative measurements in 

 inches and the configuration of the dorsal contour, in seventeen 

 examples, all diff'ering strikingly from each other. 



Spe- 

 cimen, 



Height. 



Length. 



Breadth. 



Form of 

 hinder dorsal inter~ 

 amhulacral area. 



Contour of 

 the dorsal surface. 



1 







Ol 0 

 ^T2- 



Rounded 



Very wide and 

 flattened. 



2 





<^T2 



9 7 



Carinated 



Wide and flat. 



