466 



NA TURE 



[March 20, 1890 



that of the five-toed Phenacodus (see Nature, vol. xl. 

 p. 57). Till quite lately, however, -we. have been unable 

 to follow the mode of evolution of the more complicated 

 forms of molar teeth from a common generalized type, 

 although Prof. Cope, by his description of the so-called 

 " tritubercular " type of molar structure, paved the way 

 for the true history of this line of research. 



The common occurrence of this tritubercular type of 

 dentition among the mammals of the Lower Eocene at 

 once suggests that we have to do with a very generalized 

 form of tooth-structure ; and by a long series of observa- 

 tions Prof. H. F. Osborn, of Princeton, New Jersey, has 

 succeeded, to a great extent, in showing how the more 

 complicated modifications of molars may have been 

 evolved from this generalized type. These observations 

 are of so much importance towards a right understanding 

 of the phylogenetic relationships of the Mammalia that 

 a short summary cannot fail to be interesting to all 

 students of this branch of zoology. 



The tritubercular molar (Fig. A, 6), consists of three 

 cusps, cones, or tubercles, arranged in a triangle, and so 

 disposed that those of the upper jaw alternate with those 

 of the lower. Thus, in the upper teeth (Fig. A, 7), there 

 are two cusps on the outer side, and one cusp on the 

 inner side of the crown ; while in the lower teeth (Fig. A, 

 8, 2>a) we have one outer and two inner cusps. This type, 

 when attained, appears to have formed a starting-point 

 from which the greater number of the more specialized 

 types have been evolved. The Monotremes, the Eden- 

 tates, perhaps the Cetaceans, and the extinct group of 

 Multituberculata {Plagiaulax and its aUies), must, how- 

 ever, be excepted from the groups whose teeth have a 

 tritubercular origin. 



It appears probable, indeed,that "trituberculism," as this 

 type of tooth-structure may be conveniently termed, was 

 developed from a simple cone-like tooth during the 

 Mesozoic period, and that in the Jurassic period it had 

 developed into what is termed the primitive sectorial 



Pig. a. — Types of Molar Teeth of Mesozoic Mammals. 1-5, Triconodont Type (i, Droiiiailicrium ; 2, Alicroconodon ; 3, Amphilesies ; 4, Phascolotheriuiii ; 

 5, Triconodon). 6, 7, 10, Tritubercular Type (6, Peralestes ; 7, Spalacotlierium ; 10, Asthenodoti). 8-9, 11-15, Tuberculo-Sectorial Type (8, 

 Ainphitherium ; 9, Peramns ; 11, Dryolestes ; 12, 13, Aiiihlotherinm ; 14, Achyrodon ; 15, Kiirtodon). 6 and 15 are upper, and the remainder 

 lower molars, pa, paraconid ; pr, protoconid ; >//e, metaconid ; hy, hypoconid. In the upper teeth the termination ends in cone. 



type (Fig. A, 9). The stages of the development of 1 

 " trituberculism " may, according to Prof. Osborn, be | 

 characterized as follows : — 



(i) The Haplodont type. — This is a hypothetical type ; 

 at present undiscovered, in which the crown of the tooth i 

 forms a simple cone, while the root is probably in most \ 

 cases single, and not differentiated from the crown. | 



(a) The Protodont sub-type. — This sub-type is a slight 

 advance on the preceding, and is represented by the 

 American Triassic genus Dromatherium. The crown of | 

 the tooth (Fig. A, i) has one main cone, with fore-and-aft 

 accessory cusps, and the root is grooved. 



(2) The Trico7todont type.— In this Jurassic type the 

 crown (Fig. A, 4, 5) is elongated, with one central cone, 

 and a smaller anterior and posterior cone situated in the 

 same line ; the root being differentiated into double fangs. 

 Triconodon, of the English Purbeck, is the typical 

 example. 



(3) The Tritubercular type. — In this modification the 

 crown is triangular (Fig. A, 7), and carries three main 



cusps or cones, of which the central one is placed in- 

 ternally in the upper teeth (Fig. A, 6), and externally in 

 the lower molars (Fig. A, 7). The teeth of the Jurassic 

 Spalacotheriu7n are typical examples. In the first and 

 second types the molars are alike in both the upper and 

 lower jaws ; but in the third or tritubercular type, the 

 pattern is the same in the teeth of both jaws, but with the 

 arrangement of the homologous cusps reversed. These 

 features are exhibited in Fig. B. 



These three types are regarded as primitive, but in the 

 following sub-types we have additional cusps grafted on 

 to the primitive tritubercular triangle, as it is convenient 

 to term the three original cusps. 



(a) Tuberculo-sectorial sub-type. — This modification of 

 the tritubercular type is found in the lower molars, like 

 those of Didelphys. Typically the primitive tritubercular 

 triangle is elevated, and the three cusps are connected by 

 cross ridges, while a low posterior talon or heel is added 

 (Fig. A, 9). This modification embraces a quinque- 

 tubercular form, in which the talon carries an inner and 



