Geology. 547 



As a result of a study of the Mammalia of the Atlantosaurus 

 beds, including the two forms Dryolestes and Triconodon, upon 

 which the trituberculists especially relied to prove the theory, 

 Mr. Gidley has arrived at the following conclusions : 



"(1) That the evidence obtained from the Mesozoic mammal 

 teeth furnishes no support to the tritubercular theory in so far as 

 it involves the position of the protocone and the derivation of 

 the trigonodont tooth from the triconodont stage through the 

 shifting of the lateral cones outward in the upper molars and 

 inward in the lower molars. 



" (2) That it supports entirely the embryological evidence that 

 the primary cone is the main antero-external cusp, or paracone, 

 having retained its position on the outside in most upper molars. 



" (3) That it agrees in the main with Huxley's l premolar- 

 analogy ' theory ; as supported by Scott. 



" (4) That the molars of the multituberculates, Triconodon, 

 Dryolestes, and Dicrocynodon, were apparently derived inde- 

 pendently from the simple reptilian cone ; hence the supposition 

 follows that the trituberculate type represents but one of several 

 ways in which the complex molars of different groups may have 

 been derived. 



" (5) That in the forms derived from the trituberculate type of 

 molar the order of the succession of the cusps is not the same in 

 all groups, and apparently homologous elements are sometimes 

 developed from different sources. Hence it follows that no 

 theory involving an absolute uniformity of succession in the 

 development of complex molars will hold true for all groups of 

 mammals.'''' 



Gidley urges that, though founded upon mistaken homologies, 

 Professor Osborn's convenient nomenclature be retained for the 

 molariform teeth on the ground of clearness and brevity and 

 because of its wide use in many publications. r. s. l. 



10. The Origin of Birds ; by W. P. Pycraft, A.L.S., F.Z.S. 

 Knowledge and Scientific News, Vol. Ill, New Series, Sept., 1906, 

 p. 531. — In this article Mr. Pycraft makes a very interesting fore- 

 cast of the probable appearance of the ancestral bird or pro- 

 avian. This he considers much more primitive than Archseop- 

 teryx from the Jura, for that genus is a genuine bird though 

 with several reptilian characteristics. 



The principal features of the pro-avian are thought to Have 

 been as follows : Of small size, probably arboreal, with the trunk 

 relatively longer than in modern birds. Pycraft supposes that 

 the creature had substituted leaping for climbing about the trees ; 

 from this, leaping from tree to tree would naturally follow. This 

 would in turn throw the most work upon the three inner digits of 

 the grasping hand, which would as a consequence grow stronger 

 and at the same time the outer fingers would suffer a correspond- 

 ing diminution. Correlated with this would be the development 

 of two folds of skin, one in front of and one behind the arm, and 

 the flexion of the limb and development of its post-axial border 



