160 On Cyrtoma, a new genus of Fossil Echmida. 



perforated, and hence called anambulacra, five in number; 

 each of these spaces is also composed of two vertical co- 

 lumns of large plates, equal in breadth to two of the small 

 series, and placed so that the horizontal axis of each small 

 plate is opposed alternately to the axis of the large, and 

 consequently also to that of the interval between the oppo- 

 site large plates, so as to interrupt the horizontal joinings. 

 Thus there are five large double series of plates, and five 

 small, which only require to be placed alternately together 

 in order to form a consistent moveable sphere, the walls of 

 which may be composed of any number of detached pieces. 

 Now as each plate in the large series is equal to two in the 

 small, consequently it requires nothing more than to preserve 

 the same principle of alternation in the axis of the different 

 plates in order to attain the end in view.* 



With regard to the growth of these animals, naturalists 

 appear to have fallen into curious mistakes, and seem to 

 think that it takes place only at the apex, by the formation of 

 new pieces in that situation, rather than an uniform augmen- 

 tation of all the different pieces composing the testa. It ap- 

 pears to have escaped preceding naturalists that the differ- 

 ent plates of the testa of Echinida are nothing more than 

 so many centres of ossification, which in large species be- 

 come obliterated, the testa consisting merely of five pieces 

 (exclusive of the oviducal plates) instead of many hundred, 

 as writers on this subject suppose. 



From an examination of different sized animals in various 

 stages of growth, I have reason to believe that the consoli- 

 dation of their testa begins at the base, in which situation 



* Domes constructed of masonry cannot exceed the hemisphere, as the gra- 

 vitation of the materials of the lower part of the fabric would be unsupported. 

 What has been hitherto impossible to human ingenuity, might be accomplished 

 from the model of the shell of an Echinus, in which we see the materials so 

 adjusted as to overcome the influence of gravitation, the most prevailing law of 

 matter. 



