• 66 



not uncommon in the Armagh limestone, having the root divided 

 into numerous fang- like lobes, as in a mammalian tooth. 9th. Glos- 

 sodus, for certain tongue-shaped teeth allied to Helodus (Ag.). 10th. 

 Climaxodus, for some palates allied to Pcecilodus (Ag.), but instead of 

 being transversely trigonal and obliquely ridged, they are equilateral, 

 and have the ridges transverse and parallel (like a flight of steps), 

 llth. Chirodus, for little hand-shaped teeth allied to the Ceratodi, 

 but distinguished by the thumb-like lobe projecting from the middle 

 of the long side, and which would prevent the union of the teeth in 

 pairs in the mouth, in the manner of Ceratodus. 12th, Petrodus, 

 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 Holoptychius 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 concentric lines of growth, 

 and having a fine cancellated structure internally, the Holoptychius 

 Hibberti (Ag.) Rhizodus fer ox, (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 //. 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- 

 servationsf. 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- 



* Published in 1824 in a work entitled R( flexions sur la Puisscmce Mo- 

 trice du Feu,hy JV1. 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 V Ecole Polytechnique, vol. 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. 



f 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 I'Insiitut, of which it constitutes the twenty- 

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

 published. 



