Cambridge Philosophical Society. 313 



small conical ridged teeth resembling limpets, common in the Der- 

 byshire limestone, but presenting, of all known fossil fish, the near- 

 est approach to the microscopic structure of the recent Cestracion. 

 It is also proposed to divide the genus Holoptyciiius of M. Agassiz ; 

 and instead of considering it and Rhizodus of Owen as synonymous, 

 to limit the latter to those great teeth with an elliptical section so 

 common in some parts of the Carboniferous series, accompanied by 

 large, thin, quadrate scales, marked with coocentric lines of growth, 

 and having a fine cancellated structure internally, the Holoptyciiius- 

 Hibberti (Ag.) (Rhizodus ferox, Owen) and H. Portlocki (Ag.) being 

 the types ; thus retaining the name Holoptychius for those fish so 

 abundant in the Old Red Sandstone with thick, bony, ovate, longi- 

 tudinally wrinkled scales, and minute teeth with a circular section, 

 having the H. nobilissimus, H. giganteus, &c. as the type. 



The number of new species described and figured in this paper is 

 forty-one, of which several belong to genera not previously known 

 in rocks of the carboniferous period, many showing a strong affinity 

 to the Devonian type of form. Thus we have two species of Psam- 

 mosteus, one of Chelyophorus, one (doubtful) of Coccosteus, one of 

 Asterolepis, two of Homacanthus, and one of Cosmacanthus, genera 

 hitherto only found in the Old Red Sandstone. 



On an Absolute Thermometric Scale founded on Carnot's Theory 

 of the Motive Power of Heat*, and calculated from Regnault's ob- 

 servations f. By Prof. W. Thomson, Fellow of St. Peter's College. 



The determination of temperature has long been recognized as a 

 problem of the greatest importance in physical science. It has ac- 

 cordingly been made a subject of most careful attention, and, espe- 

 cially in late years, of very elaborate and refined experimental re- 

 searches I ; and we are thus at present in possession of as complete a 

 practical solution of the problem as can be desired, even for the most 

 accurate investigations. The theory of thermometry is however as 

 yet far from being in so satisfactory a state. The principle to be 

 followed in constructing a thermometric scale might at first sight 

 seem to be obvious, as it might appear that a perfect thermometer 

 would indicate equal additions of heat, as corresponding to equal 

 elevations of temperature, estimated by the numbered divisions of 

 its scale. It is however now recognized (from the variations in the 

 specific heats of bodies) as an experimentally demonstrated fact that 



* Published in 1824 in a work entitled Ri'Jteiions sur la Puissance Mo- 

 trice du FeUyhy M. S. Carnot. Having never met with the original work, 

 it is only through a paper by M. Clapeyron, on the same subject, published 

 in the Journal de FEcole Poly technique ,\o\. xiv. 1834, and translated in 

 the first volume of Taylor's Scientific Memoirs, that the author has become 

 acquainted with Carnot's theory. — W. T. 



+ An account of the first part of a series of researches undertaken by M. 

 Regnault by order of the French Government, for ascertaining the various 

 physical data of importance in the Theory of the Steam-Engine, is just 

 published in the Memoires de VInslitut, of which it constitutes the twenty- 

 first volume (1847). The second part of the researches has not yet been 

 published. 



X A very important section of Regnault's work is devoted to this object. 

 Phil, Mag. S. 3. Vol. 33. No. 222. Oct. 1848. Y 



