208 LECTURES ON 



same way, but it is seldom that they are so distinctly marked. This 

 affords a safe clue to the true affinities of certain Aviculoid shells 

 found in the paleeozoic rocks, among the earliest forms of life in the 

 Lamellibranchiate or Plate-gilled class. They are uniformly nacreous 

 within, and it is found that the pearly lustre is due to the minute and 

 irregular corrugation of the extremely thin film of animal matter 

 which separates each layer of the shell ; this membrane preserving 

 the same pearly lustre after the shelly matter has been removed by 

 acid, but losing it when the corrugations are pressed out between two 

 pieces of glass.* 



Another very rare and remarkable bivalve found sparingly on the 

 Thorn-Oysters, was the Placunanomia pernoides, a transition form 

 between the true adhering oysters and those which fasten them- 

 selves by a solid plug passed loosely through a round hole in the 

 shell. I had long known this species, (which is so different from any 

 other that a distorted individual was described as a new genus by 

 Dr. T. E. Gray,) having observed it on the back of some very large 

 oysters, of which a large supply was sent to the Bristol Institution, by 

 the captain of a ship engaged in the West African trade. There is 

 no doubt whatever that they came from the Senegambian coast. The 

 oyster itself, (0. iridescens, Gray,) possesses very distinctive charac- 

 ters—a rare thing in that genus ; and other specimens from the same 

 coast are preserved in the British Museum. To the disturbance of 

 the prevailing theories on geographical distribution, I found the same 

 gigantic oyster among the Mazatlan shells, accompanied by the same 

 Placunanomia, and by a boring mussel, (Lithophagus aristatus.) which 

 is abundant on the warmer western coasts of Europe and Africa. I 

 believe that neither of these shells are found in the Caribbean sea ; 

 and this is only confirmatory of other evidence, that just as some forms 

 of life are peculiar to islands, not being found on contiguous continents; 

 so others may have been created with a special adaptation to coasts 

 facing the west, while others prefer the currents from the east. Those 

 to whose great labors and critical acumen we owe the present state 

 of our knowledge on the distribution of forms of life in time and 

 space, hftve perhaps sometimes considered principles as established 

 which rest on, as yet, insufficient data. It becomes us to pause before 

 we arrive at conclusions. One little fact, like the finding of the fossil 

 Malea ringens on the Atlantic side of Central America, may open the 

 door to a new course of research, rich in results, important alike to 

 geology and to recent zoology, and at the same time close it to very 

 ingenious theories that have before been considered unassailable. 

 Our principal duty, in the present state of our knowledge, at any rate 

 in Malacological science, is the patient, thorough, and honest investi- 

 gation of facts ; guided, indeed, by previously developed theories, 

 or by those which we are ourselves eliminating, but in no sense con- 

 trolled by them. And in doing so, it is only a false modesty or reverence 

 for authority which would prevent us from following the advice 



ft Full particulars on the microscopic structu-e of shells will he found in Dr. W. B. Car- 

 penter's Report to the British Association, 1844, p. 1, et seq. 



