UNIVALVE MOLLUSCANS. 275 



seems to be also a road from this central group, 

 by a Norwegian shell described by Muller as an 

 anomalous species of limpet, but which by 

 Lamarck is considered to be a bivalve. 1 The 

 lower valve in this genus is so thin that Muller 

 overlooked it; by it the animal adheres to marine 

 bodies the upper valve, like the Patella, is 

 sub-conical with a prominent vertex, and the 

 two valves are not connected by a hinge. 



A due consideration of all these circumstances, 

 of this radiation, as it were, from a typical form 

 as a centre, by various roads towards different 

 tribes, seems to prove, and the observation is con- 

 firmed by facts in other departments of nature, 

 that the world of animals, as well as that of 

 heavenly bodies, consists of numerous systems 

 each, so to speak, with its central orb, and all 

 concatenated, and revolving as it were wheel 

 within wheel, and all tending towards or branch- 

 ing from a common centre. It seems, in the 

 present instance, taking the group expressed by 

 Patella of Linne as the common centre, that 

 from thence, though by different and diverging 

 routes, we may arrive at almost every molluscan 

 group or tribe. 



The Molluscans that we have hitherto been 

 considering, with the exception of the herbi- 

 vorous chitons, derive their nutriment from the 



1 Orbicula Norweyica. 



