452 H. F. Osborn — Mammalia in North America. 



triangular plan ; these teeth have only the first lobe and half 

 the second. The upper molars of Hipparion and Coryphodon 

 illustrate the advantages of this new system of comparison 

 and of terminology. 



Scott has made a further advance in Odontology by work- 

 ing out the laws of premolar evolution or cusp addition. In 

 many groups we know that from one to four of the premolars 

 gradually acquire the exact form of the molars in order to 

 further increase the grinding surface, and we should a priori ex- 

 pect that the cusps would be added in the same order, and there- 

 fore be homologous with the molar cusps. This, as Schlosser 

 and myself had observed, is not the case. Scott shows the 

 order of cusp development in the premolars is very nearly the 

 same in all the mammals, and yet is entirely different from the 

 order followed in the molars. This law again unexpectedly 

 ties the clawed and hoofed mammals together ; the sequence 

 of cusps in palingenesis is similar to that observed by Taeker 

 in embryogenesis, and Scott is justified in proposing a new 

 terminology (protocone, deuterocone, tritocone, etc.) for the 

 premolar cusps, which will in the end prove to be a great con- 

 venience. 



I alluded above to the well known extreme and very confus- 

 ing similarity of the tritubercular molars in the early stages of 

 divergence. Trituberculism is at once the cause of clearness 

 and of doubt when we get back to the stem mammals of 

 widely different phyla. This has led to strange misconceptions 

 of phyletic affinities as exemplified in Filhol's division " Pachy- 

 lemuriens" a supposed mixture of lemur and ungulate stock. 

 There was never any such mixture^ and the question comes up 

 how to distinguish unlike forms with like teeth ? I have pro- 

 posed to make use of a dental curve which will express the 

 incipient atrophy of some parts, and hypertrophy of other 

 parts of the series, a metatrophism which will naturally termi- 

 nate in the reduction of some teeth, and excessive develop- 

 ment of others. This has not been by any means fully worked 

 out, but I believe it will prove to be of great service in direct- 

 ing attention to some of the initial tendencies of divergence, 

 which are not expressed either in the dental formula or in the 

 patterns of the teeth. Below are some of these curves. When 

 worked out by the composite method, we will find certain pri- 

 mary curves characteristic of the ordinal divisions, and minor 

 curves distinguishing the lesser divisions. Of course the laws 

 of parallelism will also be found in force here ; flesh-eating, 

 insect-eating, and grass-eating animals will be apt to have 

 similar curves even when evolved in different groups, but here 

 the dental formula and succession will come to our aid. 



