SEA-TJECHINS AND SEA-CUCUMBEKS. 32Y 



tips are larger and cut into subordinate teeth of exqui- 

 site minuteness. 



We have here an opportunity of seeing that the oval 

 or 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- 

 sponding with the marks in question ; so that the struc- 

 ture 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. 1/ridens, 

 treated vrith 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 two 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 JE. miliaris ; for, in the cor- 

 responding organs of E. sphcera, both sides of the trig- 

 onal base appear untoothed, except close to the bot- 

 tom, where a deep notch indents each margin. 



Yiewed from beneath, the head assumes an outline 

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

 of the triangle has a very obtuse projecting angle in 



