SEA-rRCHINS AND SEA-CUCUSnJEES. 327 



tips are larger and cut into subordinate teeth of exqui- 

 site minuteness. 



AVe have here an opportunity of seeing that the oval 

 01 square markings, which are thickly placed through- 

 out the calcareous substance of the blades, are certainly 

 cavities in it; for in those examples in which the pins, 

 which are very brittle, are broken, the edge of the frac- 

 ture is not even, but jagged with holes exactly corre- 

 s[)onding with the marks in question ; so that the struc- 

 tiiie is the same as that of the spines and of all the 

 other solid parts of the Urchin. 



We will now examine some specimens of P. tridens, 

 treated with potash, which will enable us to see the cal- 

 careous support better. The head-blades expand at the 

 base into three-sided prisms or pyramids, each of the 

 two interior sides of which is indented with a large 

 cavity, leaving a projecting dividing ridge, armed with 

 teeth somewhat remote from each other. The one ex- 

 terior angle is toothed in a corresponding manner, but 

 the opposite angle appears plain. The angle of one 

 blade-base fits into the cavity of its neighbour ; and, so 

 far as I have observed, when the tw^o edges thus over- 

 lap, it is the toothed one that is on the outside. Look- 

 ing from the circumference towards the centre of the 

 head, it is the left angle that is toothed and external, 

 the right being plain and sheathed. This observation, 

 however, applies only to E. miliaris / for, in the cor- 

 responding oi'gans of PJ. sjyhcera, both sides of the trig- 

 '>ual base appear untoothed, except close to the bot- 

 ■•om, where a deep notch indents each margin. 



Viewed from beneath, the head assumes an outline 

 which is rondo-triangular ; but yet such that each side 

 ■>i the triangle has a very obtuse projecting angle in 



