Royal Society, w 533 



pidius granulosus, HoloptycJdus, Diphpterus and Megalic/Uhys, Xt^,;^ 

 whicli the ganoid and placoid forms merge in one anotlier. . ,, 



That ganoid scales consist of variously modified osseous lamellae, 

 the result of successive additions made chiefly to the lower surface . 

 of each ; but also, under particular circumstances, either to a partnH 

 or to the whole of the upper surface. 



That these lamellte have not been the result of any process of 

 excretion, or depositions from a secreting surface, as supposed by 

 M. Agassiz, but that they have been formed by the calcification of 

 the lower laminae of an investing vascular periosteum ; and that 

 consequently the phagnomena attending the structure and growth of 

 these ganoid scales contribute in a material degree to establish the 

 correctness of the views recently promulgated by Professor Sharpey 

 respecting the growth and development of human bone ; the gra- 

 dual formation of Haversian canalsj being traced with great ease 

 from the simple laminae seen in the scales of Lepidosteus, Lepidotus, 

 &c., through Aspidorhynchtis, Acipenser, Holoptijchius, &c. to their 

 high degree of development in Megalickthys. 



That the study of the microscopic structure of the dermal ap- 

 pendages offish may, when carried on with due caution, be made a 

 valuable auxiliary, both in distinguishing between allied species, and 

 in establishing the existence of important affinities, even when ap- 

 plied to otherwise insignificant fragments ; but that it is capable of 

 being overstrained, and of leading to erroneous conclusions, if any 

 classifications are founded upon it irrespective of the other portions 

 of the fish to which the scales belong, because of the unequal ratio 

 in which the various parts of an organism may have been developed. 

 Thus, whilst Lepidosteus osseus presents one of the simplest forms 

 of ganoid scales, it has the concavo-convex vertebral articulations of 

 the Ophidians ; on the other hand, in many species, as in Megalick- 

 thys and Holoptychius, whilst the structure of each part of the exo- 

 skeleton is highly developed, the vertebras appear to have the double 

 concave articulation common amongst fish and enaliosaurs. 



The author, in coe elusion, acknowledges his obligations to Sir 

 Philip M. de Grey Egerton, M.P., Dr. Mantell, Mr. Binney, Mr. 

 J. E. Gray and Mr. Searles Wood, for their valuable co-operation 

 in supplying many important specimens for examination. 



** On the Mechanical Equivalent of Heat." By J. P. Joule, 

 Cor. Associate R. Acad. Sciences, Turin, &c. Communicated by 

 M. Faraday, D.C.L., F.R.S., Foreign Memb. Acad, of Sciences, 

 Paris, &c. 



After passing in review the experimental researches of Rurnford, 

 Davy, Dulong, Faraday, and others who have successively discovered 

 facts tending to prove that heat is not a substance, but a mode of 

 force, the author mentions the papers he has already communicated 

 to the Royal Society, and published in the Philosophical Magazine, 

 in which he has endeavoured to show that in the production of heat 

 by the expenditure of force, and vice versa, in the production of 

 force by the expenditure of heat, a constant relation always subsists 

 between the two. This relation he denominates the '* Mechanical 



