318 



SCIENCE. 



[N. S. Vol. VIII. No. 193. 



water or some other chemical alteration in 

 the relative amount of alkalines and salts. 

 The changes in the ammonites, though 

 more remarkable, are similar to the aber- 

 rations observable in the shells of the upper 

 and later layers of the Steinheim deposits, 

 made known to us by Hilgendorf, Sand- 

 berger, and more especially by the detailed 

 and masterly researches of Professor Hyatt. 

 In this case the Miocene Tertiary 

 Planorbis hvvis was supposed to have been 

 carried into a new lake, before untenanted 

 by these shells. Although from some un- 

 known cause the lake was unfavorable to 

 the production of normal IcEvis, whose de- 

 scendants show the results of accidents and 

 disease, yet, owing to isolation, which pre- 

 vented intercrossing with the present stock, 

 and to the freedom from competition, the 

 species was very prolific, and the lake be- 

 came stocked with a multitude of more or 

 less aberrant forms constituting new species. 

 Some of them are nearly normal, with aflat 

 spire ; others are trochiform, and others 

 entirely unwound or corkscrew- shaped. 

 Similar aberrations occur in Planorlis com- 

 plaiiatus, living in certain ponds in Belgium 

 (Magnon) ; in the slightly twisted Pla- 

 norbid, Helisoma plexata Ingersoll of St. 

 Mary's Lake, Antelope Park, Colorado, and 

 in the unwound forms of Valvata first found 

 by Hartt in Lawlor's Lake, near St. John, 

 New Brunswick, and described by Hyatt.* 

 In all these cases of parallelism or con- 

 Yergence the aberrations seem to have been 

 due to some unusual condition of the water 

 adverse to normal growth. Hence it is not 

 impossible that the singular uncoiled or 

 straight forms assumed by certain of the 

 ammonites when on the verge of extinction 

 were likewise cases of convergence and due 

 to weakness or senility, or at least to an un- 

 usual and unfavorable condition of the seas 

 in which they lived. 



♦Annual Report of Hayden's U. S. Geol. and 

 Geogr. Survey of the Territories. 



The physical causes of extinction of the 

 Mesozoic reptiles may also have been due 

 to or connected with the changes of coast 

 level, although signs of weakness and senil- 

 ity are exhibited by these. In the Como 

 or Atlantosaurus beds referred by Scott to 

 the Lower Cretaceous rather than Jurassic, 

 the ichthyosaur {Sauranodon nutans) was 

 toothless, while the colossal Cretaceous 

 pterodactyle, Ornithostoma (Pteranodon), 

 was entirely toothless. 



The colossal Pythonomorpha, offshoots of 

 terrestrial lizards, but with paddles adapt- 

 ing them for marine existence, succeeded 

 the plesiosaurs, and may have materially 

 aided in their extinction. Hence arises the 

 question : Did the extinction of the marine 

 reptiles result in or contribute to the great 

 increase of teleost fishes ? 



Before the dinosaurs began to die out the 

 type in part became specialized into lizard- 

 like tree-climbing forms and agile bird-like 

 forms. The first birds of the Cretaceous 

 were toothed, carinate, highly predaceous 

 forms, with a retrogressive side-branch of 

 wingless diving birds, represented by the 

 colossal Hesperornis, but in this case the 

 loss of teeth was undoubtedly a gain to the 

 type, compensation for the lack of a dental 

 armature in the seed-eating birds being 

 shown in the elaboration of a gizzard. 



5. GEOLOGICAL CHANGES IN THE TEETIAKY. 



Here again we have, as in former periods^ 

 a succession of earth-movements, subsi- 

 dences in one region and elevations in. 

 another, though apparently more limited in 

 extent than before, the oscillatory move- 

 ments being rather confined to coastal 

 areas, and involving adjacent shallow seas, 

 there being frequent alternations of marine 

 with brackish and fresh-water beds. As 

 Kayser remarks, the Tertiary deposits " no 

 longer extended unaltered over whole coun- 

 tries like those of older systems, but gen- 

 erally occupied only smaller basins or bay- 



