TIIANSACIIONS OF SECTION C. 645 



•which he described thus: — 'Par leur taille elles tienneiit le milieu entre les 

 Microdon et les Sphserodus. De formes arrondies et cylindracoes vers la base, les 

 dents ont leur courouue renflee en forme de massue, et siir le milieu de la couronne 

 s'(5leve encore un petit mammelon tronqu^, ce qui a valu Ti ce genre son nom de 

 Colobodus.'' 



Since that time teeth of a similar generic character have been described or 

 figured by various authors, e.g. Count Miinster (assuminp; Asterodon to be identical), 

 Plieninger, Giebel, Gervais, Meyer, Chop, E. E. Schmid, Alberti, Eck, Winkler, 

 Giirich, W. Dames, and A. S. Woodward. The typical teeth, however — i.e. those 

 upon which the ' nipple,' or apical tubercle, is present — must be sought amongst 

 the various species of Colobodus and Lepidotus (of Plieninger, 1847) ; whilst inter- 

 mediate forms, or those from which the 'nipple' has been partly or entirely 

 removed by wearing or by post-mortem abrasion, must be sought amongst those 

 described under the various species of Lepidotus, Sphccrodus, Gyrodus, ' Tetra- 

 gonolepis ' (of Winkler, and of Agassiz in part), Tholodus and Thelodus, Supleu- 

 rodus, Sargodon (not cutting teeth), and even amongst teeth variously attributed 

 to Saurichthys and to ' Saurians,' whilst the chisel- shaped, or pre-m axillary, teeth 

 are probably those attributed to Sargodon tomicus. 



Fragments of the head and trunk and scales of Colobodus have been described 

 or figured by II. B. Geinitz, Meyer and Plieninger, Giebel, Meyer, Quenstedt, Eck, 

 Kner, H. Kunisch, W. Dames, J. von Rohon, and A. S. Woodward, and must be 

 sought amongst the various species ascribed to Gyrolepis and Amblyptcrus, Lepi- 

 dotus, Heterolepidotus, Eugnathus, Pleurolepis, Dactylolepis, and also amongst 

 various Ganoid scales (' Ganoidschuppen ' and ' Fischschuppen '). 



Up to the present neither the teeth nor the scales of Colobodus have been recog- 

 nised as such in Britain by any authors, or, above the Muschelkalk and Lettenkohle, 

 abroad: its occurrence and recognition, therefore, in the Rhfetic of Britain is 

 interesting, and the author exhibited typical and transitional teeth which he found 

 and recognised in the Rhjetic 'bone-beds' of Watchet and Aust Cliff; worn and 

 abraded teeth (' Sargodon tomicus ' and ' Sphcerodus ') from thence and from 

 Leicestershire ; and what are probably the larger cutting teeth from Aust and 

 Leicestershire ; also line characteristic scales and (.'' head-) bones showing vermicu- 

 lated sculpture from Aust. All may, for the present, be referred to Colobodus 

 maxinius (Quenstedt) . 



Finally, should Colobodus prove to be identical with Lepidotus, a further fusion 

 of Heterolepidotus and Eugnathus will give Colobodus a more extended upward 

 range than has hitherto been supposed. 



5. Beport of the Committee on Earth Tremors. — See Reports, p. 333. 



6. Report of the Committee on the Volcanic Phenomena of Vesuvius. 

 See Reports, p. 312. 



JUONBAT, AUGUST 24. 

 The following Papers and Reports were read : — 



1. The Cause of an Ice Age. By Sir Robert Ball, F.B.S. 



The ordinary statement of the astronomical theory of the Ice Age seems to be 

 founded on a passage in Sir John Herschel's outlines of Astronomy. It is from 

 this that Dr. CroU's theory has been developed. It is the object of this communi- 

 cation to point out that by what seems to have been a mathematical mistake on 

 the part of Herschel, a wholly erroneous statement of the matter was presented. 



