R. F. Osborn — Original Tritubercular Theory. 323 



have every reason to believe is homologous with the large 

 external cusp or protoco'nid in the lower jaw. 



Secondly, that the external cusps in the superior molars are 

 depressed and comparatively small, consisting of two, more or 

 less well-defined cusps. Thirdly, that this palseontological evi- 

 dence lends no support, either in crown or fang structure, to 

 the evidence of embryology that the paracone (fpa, or antero- 

 external cusp) is the oldest cusp. Fourthly, that it lends no 

 support to the premolar-analogy theory, which was originally 

 suggested by Huxley* in his description of the teeth of the 

 Canidse in 1880, which has been supported by Scott and other 

 palaeontologists, and finally set forth with fresh arguments by 

 Dr. Wortman ; this \ premolar-analogy ' theory is to the effect 

 that the key to the past evolution of the molar teeth is to be 

 found in the subsequent or present evolution of the premolar 

 teeth, and that thus in many groups of animals at least the pro- 

 tocone occupies the same position in the upper molars as in 

 the upper premolars. Fifthly, that all the known Upper Cre- 

 taceous mammals with triangular molars accord with the Upper 

 Jurassic mammals in exhibiting the antero-internal cone as the 

 main cone. Finally, that no such variations of structure are 

 observed in the upper molars of the most primitive mammals as 

 would be the case if there had been different modes of origin 

 of the triangular or tritubercular crown. 



In a succeeding article I shall take up and discuss some of 

 the other points and theories raised in Dr. Wortman's interest- 

 ing and important papers. 



EXPLANATION OF PLATE XXI. 



Figure 1. —Superior molars of Dryolestes Marsh. A. Series of the left side, 



external and crown views. B. Series of the right side, external, 



crown and internal views. Yale Museum. 



pr, pr, pr, main internal cusps believed to be protocones. 



fpa, fme, smaller external cusps believed to be para- and meta- 



cones. 



c, c, c, external and internal cingula. 



i. o. f. infraorbital foramen. 

 Figure 2. — Superior molars of Peralestes Owen. Right side. External, 



oblique and crown views. British Museum. 



mts, metastyle. Other abbreviations as in fig. 1. 

 Figure 3. — Superior molars of Kurtodon Osborn. Left side. British 



Museum. 



* Collected Memoirs, vol. iv, p. 450. 



