162 THE DEPTHS OF THE SEA. [CHAP, iv 



to the mouth the distinction between ambulacral 

 and interambulacral arese is apparently lost, and 

 the sutures between the plates can scarcely be made 

 out ; the pore arese 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 

 areolse 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 areolse. 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,' and have the normal 

 number and arrangement of the principal parts. 

 They resemble the Cidaridse in the continuation ot 

 the lines of ambulacral pores over the scaly membrane 

 of the peristome to the mouth, and they approach 

 the DiadematidaB in their hollow spines, in the form 

 of their small pedicellarise, 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 areae, to the widest extenl 

 compatible with belonging to the same sub- order. 



Many years ago Mr. Wickham Flower of Par! 

 Hill, Croydon, procured a very curious fossil froir 

 the upper chalk of Higham near Rochester. It con 

 sisted of a number of series of imbricated plate; 

 radiating from a centre, and while certain sets of thesi 

 plates were perforated with the characteristic doubL 



