2 



BRITISH FOSSILS. 



semiglohus of Munster) we see a species which is truly intermediate 

 between such a Galerites as G. castaneus and the more typical forms of 

 Dijsastev. 



British specimens of this Dysaster vary in outline from nearly com- 

 pletely circular to subpentagonal, with even an approach to oblong ; 

 every gradation between the two extremes of form is common. The 

 dorsal surface is very uniformly and gently convex, though varying con- 

 siderably in altitude. The highest point of the back is almost always a 

 little behind the centre ; very rarely a specimen is met with having it 

 still further back. The sides are tumid, and in the subpentagonal 

 specimens, slightly and very obtusely angulated by the prominence of the 

 interambulacral spaces. The ventral surface is concave in the region of 

 the mouth, very convex, and almost nodulose in the interambulacral 

 spaces ; the ambulacral areas being depressed. The odd or anal inter- 

 ambulacral area is peculiarly prominent and gibbous. Its extremity is 

 squared by the inferior terminations of two ridges which bound the 

 conical groove, in the upper part of which, nearly, but not quite on a 

 level with the dorsal surface of the body, is lodged the anus. 



The three anterior ambulacra converge nearly in the centre of the 

 back. The antero-laterals are separated on each side from the anterior 

 ambulacrum by a perforated genital plate ; that of the left side is granu- 

 lated in its upper part, being the madreporiform plate. Behind the summits 

 of the antero-lateral ambulacra, which curve gently forwards before they 

 terminate, are the other two genital holes. The eye-holes are very 

 small, but distinctly terminate the three anterior 'ambulacra ; the other 

 two seem to be removed from the disk to the terminations of the 

 postero-lateral ambulacra, but of this I have not clearly satisfied 

 myself. 



The ambulacra are all complete, running from the mouth to their 

 dorsal terminations ; on the back they are on a level with the inter- 

 ambulacral plates. The number of ambulacral plates exceeds that of 

 the interambulacrals by four or five times. The three anterior ambu- 

 lacra are narrower than the two posterior, which are often nearly but 

 not quite twice as broad as the former ; the proportionate width, how- 

 ever, as I have satisfied myself by measurements, is not constant. The 

 narrowest ambulacrum towards its summit is the anterior one, but it 

 widens out lower down. Each pair of ambulacral plates is perforated 

 at its outer side by a pair of pores. Near the mouth the ambulacra 

 widen, and the pairs of pores are ranked in about three oblique series of 

 three pair in each, but their relations to the plates there are obscure. 

 All the plates, whether ambulacral or interambulacral, are minutely 

 granulated, and among the granules are interspersed very small spinife- 

 rous tubercles with areolae and punctated summits ; but whether the 

 bosses on which they stand are crenulated I have not been able to mxake 



