30 



BRITISH FOSSILS. 



antres C^lacanthes C'est dans I'Old Eed et dans 



riiouille que la famille des Celacanthes acquiert son plus haut degre 

 de developpement ; passe cette epoque elle decline rapidement, et son 

 dernier representant, qui d'ailleurs est fort douteux, appartient a la 

 craie." 



In 1849 Professor "W. C. Williamson gave an account of the 

 stracture of the scales and the walls of the ossified air bladder of 

 Macropoma in his valuable memoir On the Microscopic Struc- 

 ture of the Scales and Dermal Teeth of some Ganoid and Placoid 

 Fishes," published in the Philosophical Transactions for that 

 year. 



Professor Williamson shows that the tubercles which ornamei\t 

 the scales and opercular plates of Macropoma are composed of 

 layers of kosmine coated with ganoin, and resting, in the case of 

 the operculum, upon lamellae of bone, in that of the scales, upon a 

 lamellar substance which contains no lacunse, but presents layers 

 of irregular tubes interposed between the lamellas. Successive 

 layers of the tubes cross one another in direction. 



At the base of each tubercle, whether on the operculum, or on 

 the scale, there is a cavity, which communicates by one or more 

 canals with the exterior. 



The walls of the structure called air bladder " by Mantell* 

 (who seems to have considered the solidity of the parietes of this 

 organ to result from mineralization) and " stomach " by Agassiz, 

 are proved by Professor Williamson to be composed of lamellae 

 between which are developed large lacunae, identical with those 

 found in the endoskeleton of the fish. Some of the external 

 lamellae lose their exact parallelism with those below, and one in 

 particular assumes an undulating arrangement. On both sides of 

 the folds of its undulations large irregular lacunae are placed. 

 This again is invested by other dense and apparently structureless 

 lamellse, which fill up the inequalities of the undulating layer, and 

 form the external smooth surface of the organ. 



With respect to the functions of this apparatus. Professor 

 Williamson remarks : — 



" I am disposed to believe that it has been an organ fulfilling the 

 functions of an air bladder. Its osseous structure would render it 

 capable of resisting a considerable amount of pressure ; and if its patu- 



* In the Medals of Creation, 1 844, Mantell gives up his first interpretation, and 

 adopts that of Agassiz, out in " The Petrefactions and their Teachings," 1851, p. 437 

 he writes : — " Air Bladder (or Stomach?^ of the Macropoma. — In every example of 

 " this fossil fish that I have dissected,;th ere lies within the body, generally nearest the 

 " upper or dorsal part of the cavity, a long hollow cylindrical substance, often 

 " 7 inches in length, and inches in'diameter, covered with a thin, brittle, scaly integu- 

 *' ment, which readily separates into two or three laminap. The anterior part of this 

 *' organ, which lies close to the posterior margin of the opercular bone, is always 

 " imperfect, appearing as if torn or ruptured ; the caudal extremity terminates in a 

 " cul-de-sac. From the situation and structure of this viscus I supposed it to be an 

 " air bladder, and described it as such in the " Fossils of the South Downs," in 1822, 

 " but on Professor Agassiz's visit to my museum at Brighton, that eminent naturalist 

 " pronounced it to be the stomach. Recent microscopic investigations of the invest- 

 *' ing integument have, however, tended to estabUsh the correctness of my original 

 " interpretation of the nature of this remarkable body." 



