Notices of Memoirs — Dr. C roll's Theory Criticised. 171 



IV. — "Ueber den Hautschild eines Eochen atjs dbr Marinen 

 Molasse." By Prof. A. Baltzer. Mittheil. Naturf. Ges. Bern, 

 April, 1889. 



AN unusually large dermal tubercle of a Bay from the Molasse of 

 Miigenwyl, Canton Aargau. is described and figured by Prof. 

 Baltzer. The specimen measures about OOo in length, is of oval 

 outline, and appears to consist of four broad, conical tubercles fused 

 together. A microscopical section proves the Selachian nature of 

 the fossil, and it seems to be related to the dermal tubercles already 

 described under the names of Acanthobatis and Dynatobatis. The 

 author does not suggest a generic or specific determination ; but 

 a postscript states that Dr. Jaekel regards the specimen as referable 

 to the genus Trygon, and will shortly describe this with other evidence 

 of the same fish from the Swiss Molasse. 



V. —A Criticism of Dr. Croll's Theory of Alternate Glacial 

 and Warm Periods in each Hemisphere, and of Interglacial 

 Climates. By H. H. Howorth, M.P., F.S.A. (Memoirs Man- 

 chester Lit. and Phil. Soc. ser. 4, vol iii. 1890.) 



BRIEFLY stated, the conclusions to be drawn from Dr. Croll's 

 theory, are " that there has been throughout geological time 

 an alternate glaciation of each hemisphere, ultimately caused by 

 changes in the eccentricity of the earth's orbit, and directly engen- 

 dered by the greater amount of warm water forced into each 

 hemisphere by the alternately greater potency of the Trade winds 

 North and South of the Equator." The author grants two of Dr. 

 Croll's postulates, namely, that climate is largely dependent on the 

 distribution of ocean currents, and that these are chiefly dependent 

 on the winds; but he maintains that there is no evidence that the 

 south-east trade winds are stronger than the north-east, the fact that 

 they blow across the equator being accounted for by the situation of 

 the parallel of greatest mean heat being to the north of that great 

 circle. Here we have the largest area of land, or the " furnace " 

 which causes the winds to blow across the equator ; and thus Mr. 

 Howorth argues that the circulation of the trade winds does not 

 depend on special differences between the temperature of the equator 

 and that of the north or south poles, at different periods. 



There is no evidence to show that any great changes have taken 

 place in the relative distribution of land and water since the Glacial 

 epoch, whereby the parallel of greatest mean heat might have been 

 shifted southwards; nor can Mr. Howorth find evidence to show 

 extensive glaciation in the southern hemisphere, as compared with 

 the northern. 



The weakness of the evidence of Glacial periods in older geolo- 

 gical eras is discussed by the author, and coming to the Glacial 

 period itself, he points out the uncertainty of the evidence. <; We 

 have not yet (he says) found a key by which we can give a rational 

 explanation of the true succession of so-called glacial deposits in two 

 adjoining counties, much less in two larger geological areas." He 



