998 The American Naturalist. [December, 
made strenuous efforts to secure additional evidence, which 
have not thus far been successful. In the meantime too great 
emphasis cannot be laid upon the fact that all the existing palæ- 
ontological evidence points in the same. direction, namely, to the 
presence of the chief cone upon the inner side of the upper 
molars, and upon the outer side of the lower molars. An im- 
portant oversight on the part of those who are still uncon- 
vinced of the tritubercular theory, is the necessity of a mechan- 
ical adaptation of the upper to the lower teeth in every stage 
of development, which is perfectly met by the tritubercular 
theory. Given the universally acknowledged trigonid or tri- 
angular arrangement of cusps in the lower teeth, no mechani- 
cal relations can be imagined in an upper molar crown which 
originated with the external cusps, paracone and metacone. 
If the main object of paleontological research is to trace 
back various lines of descent as far as possible, the very unity 
of primitive type makes this apparently more difficult than 
before, but not really so. We were working before upon a 
false basis, or no basis at all; we can now advance upon the 
certain basis of primitive form and the one requisite of progress 
is to employ much more exact methods of description and 
analysis. 
1. THE THREE PRIMARY FORMS. 
So far as the molar teeth were concerned, there were, to our 
present knowledge, but three great primary forms, which suc- 
ceeded each other as stages and also persisted. From one or 
other of these all the known recent or fossil mammalian teeth 
have diverged, including probably the Multituberculates. 
These types are illustrated in the accompanying cut. First, 
the haplodont crown, which links the mammals with the rep- 
tiles; second, the triconodont crown which was predominant in 
the Lower Jurassic period ; third, the tritubercular crown which 
appeared in the Lower Cretaceous' and has been by far the 
most productive. The transitions between these great types 
1 Tt now appears advisable that the ‘so-called Como (Atlantosaurus) Beds of 
North America and the Purbeck Beds of England should be placed in the base 
of the Cretaceous instead of in the Upper Jurassic as formerly. 
