SILKY-SPINED EGG-URCHIN. 173 



apex to mouth. When fresh taken, one is much more apt to 

 confound it with a depressed variety of the common Sea- 

 Urchin, from which, however, the five pairs of pores in 

 each row at once distinguish it, independent of the excel- 

 lent characters presented by the spines. The spaces be- 

 tween the rows of pores are rather broad and very tuber- 

 cular, bearing secondary spines. The ambulacral plates 

 are rather narrow, each bears two primary tubercles, which 

 are very prominent, and polished and elevated on promi- 

 nent bases, round which a number of secondaries are 

 seen. Each inter-ambulacral plate bears a transverse row 

 of four or five primaries, with a number of secondaries 

 interspersed. The primaries diminish in number towards 

 the apex. The ovarian plates are thickly studded with 

 secondary tubercles, and the madreporiform tubercle is 

 large, prominent, heart-shaped, and compact. The spines 

 thickly stud the body, and are nearly equal in size. They 

 have a fine silky lustre, and when magnified are seen to 

 be striated longitudinally, the stria) and the intermediate 

 spaces' nearly equal in breadth and crossed by very fine 

 transverse stria?. The largest specimen measured a little 

 more than nine inches in circumference, and was one inch 

 and eight-tenths in height. 



This Sea-Urchin was first added to the British list 

 by Dr. Fleming, and afterwards found by Mr. Goodsir 

 and myself, during our visit to the Orkney and Shet- 

 land Islands in the summer of 1839. We obtained 

 three specimens of it, each of which was taken in a locality 

 distant from the other two. The first we dredged in 

 ten fathoms water, opposite the town of Lerwick ; the 

 second, off Scalloway on the west coast of the Mainland 

 of Shetland ; and the third, in rather deeper water in the 



