218 PROCEEDINGS OF THE 



Looking at the specimen in its present condition, there can be 

 no doubt that the greater part of the surface has had the spines 

 rubbed off, and at other places broken off at different lengths from 

 the base. To all appearance it has also had a new set growing on. 

 This is obvious from what we shall call the " new spines " being 

 all very small, and out of all proportion to the size of the shell or 

 " test," while the portions of the old broken spines that remain 

 are of the normal size. This forces on us the conviction that the 

 two sets are not of the same age ; the new spines being so feeble 

 and small, that we cannot believe that they grew from the 

 beginning in connection with the large tubercles on which they 

 stand. Many of the denuded tubercles bear no new spines ; but 

 on close inspection it is seen that the convexity of such tubercles 

 has been rubbed off, and it appears that the new spines are only 

 reproduced when the tubercle and membrane have not suffered 

 from abrasion. 



In regard to the growth of the " tests " of the echinidae, it has 

 been long known that the common membrane of the " test," by 

 dipping between the edges of each plate, supplies calcareous 

 material for the uniform enlargement of the whole. But the great 

 variety of the contours that may be noticed amongst specimens of 

 Echinus sjjhaera, shows that the law of enlargement does not keep 

 strictly to uniformity. In the taller examples the supply to the 

 edges of the plates must be greater to the vertical or polar edges 

 than to the lateral, and the reverse must be the case with the 

 flatter forms. 



The principle, however, cannot apply in the same way to the 

 growth of the spines ; and I am not aware that any other 

 explanation has been given. 



The mutilated specimen of Echinus sphaerct in my possession 

 strengthens an opinion which I have entertained for some years 

 past, that the spines grow or enlarge by concentric layers, as well 

 as by their longer axis. I was first led to this belief by observing 

 that in many of the glacial fossil spines, where all the soft animal 

 matter had been removed, parts of the different layers came off, 

 in some cases appearing like the drawn-out slides of a telescope. 

 Sometimes the core comes out, leaving the external walls as an 

 empty tube, as is well shown by the specimens exhibited. 



In the recent example, many of the spines that have been broken 

 have the inner layers produced beyond the outer ones, shomng 



