1G8 On Cyrtoma, a new genus of Fossil Echinida. 



reaching down to the circumference of the spheroid, the testa 

 becomes rounded, and finally assumes a spherical form. 

 Thus M. Agassiz makes the growth to be at the apex. 

 It is singular that so profound an observer should not 

 have seen that instead of a spheroid, such a mode of deve- 

 lopment as that which he describes would produce a simple 

 tube, and that as the proportion of circles are to each 

 other as the squares of their diameters, so the growth of 

 the different plates composing the shell of an Echinus must 

 be augmented rather than diminished, as they fall from 

 the apex towards the diameter of the disc. The views 

 of comparative anatomists and naturalists being thus at 

 variance with this simple law, I put the matter to the test by 

 reducing the several parts of the testa of a large Echinus 

 to a thin transparent film,* when the successive layers of 

 deposit were every where observable around the margins of 

 each plate, and this was observable in plates belonging 

 to the base as well as in those of the apex of the shell, thus 

 proving nature to be in the right, and showing the fallacy 

 of supposing that the growth of these animals takes place 

 only at the apex. With regard to the formation of new 

 plates at the apex there can be no question, but it is too 

 much to say that the number of plates in each vertical series 

 depends on the size of the animal, or that they differ so 

 much as some suppose in individuals of different sizes. In 

 five individuals of the same species of Echinus, I find 

 the smallest two inches, and the largest four and a half 

 inches in diameter, yet the largest has but eighteen, and 

 the smallest as many as fifteen plates in each correspond- 

 ing vertical series, while those of three inch diameter have 

 also eighteen, thus proving that the animal continues to 



* See figs. 6 and 7, plate III. Fig. 6 represents the structure exhibited at the 

 base, and fig. 7 at the apex of the shell. The layers of deposit are distinctly 

 seen surrounding the different plates. 



