November 7, 1902.] 



SCIENCE. 



749 



Study of the Lakes of Snowdonia and Eastern 

 Carnarvonshire ' by T. J. Jehu (Trans. Boy. 

 Soc. Edinh., SL., pt. 2, 1902, 419^67, 8 pL). 

 Twenty-six pages are given to an account of 

 the kike basins, illustrated by contoured maps 

 and true-scale sections. The lakes are of two 

 kinds: the larger ones lying in the main val- 

 leys, the smaller occupying cirques (cwms). 

 After discussing the origin of the lakes, it is 

 concluded that they are relatively subordinate 

 results of the glacial erosion by which the 

 valleys of the Welsh mountains have been 

 strongly scoured. As seems to be generally 

 the case in such regions, the main valleys are 

 preglacial, but now ' the more important val- 

 leys are at places over-deepened as compared 

 with the lateral valleys and * * * have a 

 trough-like form with flat bottom and steep 

 cliff-like walls.' Tributary streams often cas- 

 cade into the main valleys. Cirques, with or 

 without lakes, occur at the valley heads. " If 

 the glaciers have thus * * * eroded the chan- 

 nels along which they flowed, the excavation 

 of rock basins below the general level of the 

 valley floor * * * need no longer excite sur- 

 prise or be looked upon as anything more than 

 subordinate incidents in the general history of 

 ice. erosion." 



It is suggested that ' the lakes occupy in 

 their respective valleys just those positions in 

 which the glaciers might be expected to have 

 carried on most actively the work of erosion,' 

 and these positions are said to be next above 

 narrows, presumably due to harder rocks, 

 where the glacier would be retarded in its 

 flow; but this last point seems open to ques- 

 tion. The erosion of a lake basin in a valley 

 floor just above a hard-rock narrows would not 

 be inconsistent with a maximum erosion fur- 

 ther up the valley where the glacier was 

 thicker, for erosion might depend on the maxi- 

 mum pressure of the ice, rather than on its 

 retardation. The height of hanging lateral 

 valleys should be considered along with the 

 depth of lake basins in determining the places 

 of greatest glacial erosion in main valleys. 



W. M. Davis. 



RECENT ZOOPALEONTOLOGY. 



TEIASSIC lOHTHYOSAURS FROM CALIPOENIA AND 

 NEVADA. 



IcilTiiYOSAURS are so rare in America and 

 Triassic ichthyosaurs are so rare everywherfe, 

 that these discoveries in Nevada and in Shas- 

 ta County, California, are particularly wel- 

 come. Professor John C. Merriam* describes 

 very fully the Shastasaurus of the Upper Trias- 

 sic of California fi-om considerable portions of 

 seven individuals, together with many isolated 

 bones and teeth representing nearly the whole 

 of the skeleton, but lacking the very important 

 distal portions of the paddles. These remains 

 are placed in six species. From the Middle 

 Triassic of Nevada, the Cymbospondylus of 

 Leidy, including three species, is more fully 

 defined and characterized. 



RELATION OF THE OSTRACODERM AND ARTHRODIEAN 

 FISHES. 



Dr. Otto Jaekel contributes a new discus- 

 sion f of this group decidedly at variance with 

 the views of Smith Woodward and Dean. He 

 unites the Arthrodira and Ostracodermata, 

 which have been separated by Cope, Smith, 

 Woodward and Dean, into the single order of 

 Placoderms. Among the Ostracoderms he be- 

 lieves that the Pteraspids have retained a 

 larval character, whereas the Asterolepids have 

 become somewhat more specialized. The Coc- 

 costeid arthrodires including Coccosteus, Din- 

 ichthys _ and Titanichthys, have attained a 

 higher organization, and, owing to their freer 

 motions, have a completely segmented skeleton 

 provided with limbs, which enables us to com- 

 pare them with other vertebrates. He gives 

 a partial restoration of Coccosteus, the chief 

 feature of which is the prominent pelvic gir- 

 dle, the existence of which has been questioned 

 by Dean. The Coccosteids exhibit parallels 

 with the ancient tyi^es of sharks and crossop- 



* ' Triassic Ichthyopterygia from California 

 and Nevada,' University of California Publica- 

 tions, Bulletin of the Department of Geology, 

 Vol. 3, No. 4, pp. 63-108, pis. 5-18. 



t ' Coccosteus und die Beurtlieilung der Placo- 

 dermen,' Gesells. naturf. Freuvde zu Berlin, 20 

 Mai, 1902. 



A 



