Qn Cyrt na, a new genus of Fossil Echinida. 163 



*ere bent up to meet the anus, and the integuments are 

 pread ouf enclose a sphere. The arrangement of those 

 P e .°\is in the Echinus is thus in perpendicular columns 

 disposed in alternate pairs. This you can best examine 

 when the shells have been well macerated and bleached, 

 and as the numerous sutures never anchylose, the structure 

 is equally obvious in the adult and in the young animal." 

 A little further on, Dr. Grant observes, " The number of 

 columns»is very uniform in these animals, but the number 

 of similar pieces composing a column varies with the 

 species, and the age of the individual. The new pieces 

 which are added as the animal enlarges, are inserted at 

 the anal margin, where they are consequently always small, 

 and they appear not to be added at any other point. The 

 pieces are thus gradually pushed down towards the mid- 

 dle or base of the column by those which successively 

 make their appearance at the upper narrow apex of each 

 perpendicular series. From the analogy of the articulated 

 and molluscous classes I had formerly imagined that these 

 plates of the Echinoderma were exuded from the surface 

 of the skin and beneath the epidermis, and that they increased 

 in every dimension by addition of calcareous matter to their 

 inner surface and around all their margins. But the 

 homogeneous internal structure of these pieces presents 

 no trace of such successive depositions, and more closely 

 resembles the porous texture of shells which are formed by 

 one deposition, and which are periodically cast and renewed." 

 Dr. Sharpey, Professor of Anatomy and Physiology in 

 University College, London, in an article on Echinodermata, 

 Cyclop. Anat. Phys. Part ix, 1837, speaking of the skeleton 

 of sea urchins, observes that the calcareous matter is 

 disposed in polygonal plates, which being firmly joined 

 one to another, form by their union a shell approaching 

 more or less to a speroidal figure ; by this Dr. Sharpey 



