SEA-HEOHINS AND SEA-CUCITMBEES. 337 



■whicli is set on at a very oblique angle •with the axis 

 of the spine, reminding one, as we look at the spine 

 laterally, of the budding tines on the horn of a young 

 deer. 



At first, perhaps, you are at a loss to know what 

 purpose this shoulder can serve ; but by turning to the 

 shell and carefully observing the sptaes in their natural 

 connexion with it, you will observe that the obliquity 

 of its position accurately corresponds with the angle 

 which the individual spines form with the surface of 

 the shell from which they spring ; and that the shoulder 

 has its plane exactly parallel with the latter, but raised 

 a little way above it. Now the entire shell, during 

 life, was clothed with a living vascular flesh, having a 

 thickness exactly corresponding to the distance of the 

 shoulder from the shell. This shoulder, then, was an 

 attachment for the muscular bands, whose office it 

 was to move. the spine to and fro ; the projection af- 

 fording the muscles a much hettev purchase, or power, 

 than they could have had if they had been inserted into 

 the slender stem itself. 



The tubercles on the shell show a structure which 

 corresponds with this. They are very minute ; but 

 each of them is regularly formed, and is crowned with 

 its little polished nipple, on which, as I have said, the 

 spine works, as by a ball-and-socket joint. These are 

 arranged with perfect regularity in quincunx, and by 

 close examination you will see that each is inclosed in a 

 little area formed by a very low and narrow ridge of 

 the shell, which makes a network. On the lateral por- 

 tions of the under surface the meshes of this net are 

 particularly conspicuous, and we see that they consti- 

 tute shallow hexagonal cells, in the midst of which 

 15 



