1(52 THE DEPTHS OF THE SEJ. [chap. iv. 



to the mouth the distinction hetween ambulacral 

 and interambulacral area? is apparently lost, and 

 the sutures between the plates can scarcely be made 

 out ; the pore area? are reduced to mere lines of 

 double pores, and the whole of the surface of the 

 shell is studded over uniformly with the very large 

 areolae of primary tubercles, bearing spines which 

 are small and delicate and apparently quite out of 

 proportion to the mass of muscle connected with 

 them which fills the areolae. As in Calveria, the 

 tubercles are perforated. 



We have thus become acquainted with three 

 members of a family of urchins which, while differ- 

 ing in a most marked way from all other known 

 living groups, bear a certain relation to some of these, 

 and easily fall into their place in urchin classification. 

 They are 'regular echinids, 5 and have the normal 

 number and arrangement of the principal parts. 

 They resemble the Cidaridae in the continuation of 

 the lines of ambulacral pores over the scaly membrane 

 of the peristome to the mouth, and they approach 

 the Diadematidae in their hollow spines, in the form 

 of their small pedicellariae, and in the general structure 

 of the jaw pyramid. From both of these families they 

 differ in the imbricated arrangement of the plates and 

 in the structure of the pore areas, to the widest extent 

 compatible with belonging to the same sub- order. 



Many years ago Mr. Wickham Flower of Park 

 Hill, Croydon, procured a very curious fossil from 

 the upper chalk of Higham near Rochester. It con- 

 sisted of a number of series of imbricated plates' 

 radiating from a centre, and while certain sets of these 

 plates were perforated with the characteristic double 



