2 



BRITISH FOSSILS 



In the Britisli Museum are three specimens of this species from 

 Blackdown. They differ from each other considerably in form, and 

 in degree of depth of the dorsal ambulacra." 



A. 



B. 



C. 



Length - - 





Wt 



Breadth - - 1^ - 



ItV - 



1 



Height - - I-pV 



Ot4 - 



0-^ 



Anterior ambulacra (pores) 28 pairs 



26 



24 



Antero- lateral •« 43 



86 



30 



Postero-lateral - 20 



22 



16 



The specimen B. is apparently adult, but dwarfish ; and C. is a 

 very young individual with the ambulacra but slightly impressed. 



In the larger specimen the vertex is considerably behind the 

 centre ; the dorsal ambulacra are deeply impressed ; the anterior 

 sulcus becomes shallow at the margin, and is prolonged to the 

 bilabiate mouth ; the posterior end is slightly flattened, and nearly 

 vertical ; whilst in the smallest specimen it slopes at an angle of 

 73°. The vent is high up, small, and elliptical ; the plates on each 

 side of it have slight central prominences, converging to the caudal 

 protuberance, beyond which on each side is a moderately projecting 

 lateral boss. 



The anterior ambulacral hollow is wide, finely granulated and 

 tuberculate ; the pairs of pores are separated by distinct tubercles. 

 The lateral grooves are nearly smooth ; the antero-lateral pair are 

 shorter than the odd ambulacrum, and doubly curved ; the posterior 

 pair are less than half the length of the front pair. The spaces 

 between the petals are pinched up near the apical disk, and more 

 densely and roughly tuberculated. 



The apical disk is composed of four perforated genital plates, of 

 which the right^ antero-lateral includes the madreporiform body ; 

 there are five perforated ocular plates. 



The surface of the test is very finely granulated above, with few 

 scattered tubercles in the peripetal zone ; the tubercles are more 

 numerous and regularly distributed on the sides, becoming larger 

 towards the base. 



Affinities. — In its contour, and the form and proportions of its 

 dorsal ambulacra, the Hemiaster Murcliisonim agrees with the 



* Incorrectly stated to be the left in the description of Micraster cor-anguinum, 

 Decade III. 



