2 



BRITISH FOSSILS. 



vertical row of the odd ambulacrum, about 14 and 18 respectively in 

 the rows of each lateral ambulacrum, and 7 in each vertical row of 

 each lateral interambulacrum. These plates all bear minute scattered 

 primary tubercles, with interspersed granules. The margins of the 

 plates are in some examples slightly prominent, and as if marked by 

 a bordering line ; more usually they are quite plane, and occasionally 

 slightly impressed. Beneath, the mouth is placed very near the 

 anteal extremity, at about or less than one fourth of the length of 

 the test from the margin, which in the region of the odd ambulacrum 

 is slightly sinuous. The two posterior ambulacra are necessarily 

 much longer than the others below, and proceed from near the 

 hinder extremity to the mouth, their course marked by a slight 

 depression. Their plates become elongated and polygonal, and 

 though tliickly strewn with granules bear very few primary 

 tubercles. These last are more developed on the escutcheon-like 

 area formed by the ventral portion of the odd or posterior inter- 

 ambulacrum. The mouth itself is roundish or ovate, its fore lip is 

 depressed, its hinder lip tumid and elevated but not overlapping. 

 The apical disk is with difficulty distinguished from the neigh- 

 bouring plates ; it is much elongated, its genital and ocular pores 

 are minute, and the madreporiform body not very strongly marked. 

 On flint casts the four genital pores are usually strongly impressed, 

 as also are the pores of the avenues, which form homogeneous rows 

 indistinctly indicated, owing to their distance from each other, and 

 their minuteness. 



The varying proportions of this species may best be illustrated 

 by the following comparative measurements (in inches and parts of 

 inches), of six specimens of slightly different bulk. 





No. 1. 



No. 2. 



No. 3. 



No. 4. 



No. 5. 



No. 6. 



Height 





Of 









OH 



Length 



1 



OA 



0^ 



OA 



'-'to 



OU 



Breadth 



OA 



'-'l 0 



'-'l 0 





0 eh 



OA 



Height of vent 

 above margin 







'-'T2- 



12 





OA 



I believe there is little doubt of this pretty urchin being the 

 A.nanchytes pilula of Lamarck, whose brief diagnosis " A. minima, 

 " ovato-glohosa, suhtus convexiuscula, ano in summo margine" 



