On Cyrloma, a new genus of Fossil Echinida. 165 



would necessarily have been more limited, especially as we 

 have already stated, the consolidation of the plates commences 

 at the base, and slowly advances upwards towards the apex, 

 according to the age of the individual ; a circumstance 

 which may have led to the supposition that they never 

 unite in this situation, thus accounting for the discrepancies 

 on the subject. This peculiar mode of development may 

 also have led to the supposition that the growth of Echini 

 depends on the formation of new plates at the apex, from 

 which they are gradually pushed down towards the base 

 as new pieces are thrown out above, in proportion as the 

 animal enlarges, for as the plates on the upper part of 

 the disc are last of all consolidated, it is easy to imagine 

 that they are all new. Professor Agassiz carries this view 

 of the case much farther than Professor Grant, and refers 

 the difference between Echini and starfish to peculiarities 

 in the development of the new plates " since the large 

 growth of the plates in the summits of the spheroid, combined 

 with the contraction of the interradial planes would pro- 

 duce a starfish ; while, vice versa, the increase of the inter- 

 radial planes and the reduction of the central plates in the 

 starfish would produce a spheroid. Nor is this a mere 

 hypothesis ; we shall see hereafter that the essential differ- 

 ence between Echini and Asteriae consists in the different 

 modes of their growing." However important such general 

 observations may be to the comparative anatomist, whose pro- 

 vince it is to investigate the form and functions of organs 

 in beings of different and sometimes opposite characters, they 

 can only be useful to the naturalist as far as they are calcula- 

 ted to establish relations either of analogy or affinity. We 

 can appreciate the very beautiful design of Dr. Grant in his 

 lectures, of tracing the gradual advancement of animals from 

 the simplest to the most complete forms, according to the 

 more prominent characters of their organization in different 



