68 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 3, 



donsaurian affinities. Among the many remains discovered by the 

 zealous research of Dr. Dawson, I do not know that a single specimen 

 of one of the pectoral plates, so characteristic of all Labyrinthodonts, 

 has made its appearance. They may possibly have been Amphibia ; 

 but their skulls, their cycloid scales, and their deeply biconcave, 

 fish-like, vertebral centra appear to me to indicate a closer affinity 

 with the Ophiomorpha (Ccecilia, Ichthyophis, tfcc.) than with the 

 Labyrinthodontia. 



Of the unquestionable Labyrinthodonts which occur in the Car- 

 boniferous rocks, then, Anthracosaurus is the only genus regarding 

 the vertebral column and ribs of which there is any information ; and 

 the description and comparisons which I have given seem to me to 

 necessitate the conclusion that, side by side with the Archegosaurian 

 type, the Mastodonsaurian type of vertebral organization, hitherto 

 known to occur only in the Trias*, was well developed in the An- 

 thracosaurus of the Scotch coal-field. At the same time, the anchy- 

 losed condition of the neural arches, the supratemporal foramina 

 (which may, however, be parts of the 'mucous grooves ' common upon 

 Labyrinthodont skulls, the floor of which was very thin, or merely 

 membranous in the temporal region of Anthracosaurus), and the 

 strong median convexity of the snout, separate Anthracosaurus from 

 any known Triassic Labyrinthodont. And though, in the general 

 form of the cranium and in some other respects, Anthracosaurus has 

 a certain resemblance to the Permian Dasyceps, it differs as widely as 

 possible from it in its dentition. 



3. On the Thickness of the Pampean Formation, near Buenos Ayres. 

 By Charles Darwin, Esq., M.A., F.K.S., F.G.S., &c. 



M. Sotjrdeaux and J. Coghlan, Esq., C.E., have had the kindness 

 to send me, through E. B. Webb, Esq., C.E., some excellent sections 

 of, and specimens from, two artesian wells lately made at Buenos 

 Ayres. I beg permission to present these specimens to the Geolo- 

 gical Society, as they would be of considerable service to any one 

 investigating the geology of that country. The Pampean formation 

 is in several respects so interesting, from containing an extraordinary 

 number of the remains of various extinct Mammifers, such as Mega- 

 therium, Mylodon, Mastodon, Toxodon, &c, and from its great extent, 

 stretching in a north and south line for at least 750 geographical 

 miles, and covering an area fully equal to that of France, that, as 

 it appears to me, a record ought to be preserved of these borings. 

 Southward, at the Rio Colorado, the Pampean formation meets the 

 great Tertiary formation of Patagonia ; and northward, at Sta. Fe 

 Bajada, it overlies this same formation with its several extinct shells. 

 In the central region near Buenos Ayres no natural section shows 

 its thickness ; but, by the borings there made in two artesian wells 

 (figs. 1 & 2), the Pampean mud, with Tosca-rock, is seen to extend 



* Nothing is at present known of the vertebrae of Dasyceps BuckJandi, from 

 the Bunter sandstein of tins country. See Memoirs of the G-eological Survey of 

 Great Britain : — The Geology of the Warwickshire Coal-field ; by H. H. Howell, 

 F.G.S. 1859. 



