This type of tooth differs from that of all other known recent or extinct Mammals. The nearest approach to it is made by the middle true molar of Pliolophus Int1piceps, a small extinct herbivorous Mammal from the London clay (fig. 96, m, That the fragment in question is the jaw of a Mammal is inferred from the implantation of the tooth by two or more roots. Most Mammals are known to have certain teeth so implanted. Such complex mode of implantation in bone has not been observed in any other class of animals. Why two or more roots of a tooth should be peculiar to viviparous quadrupeds, giving suck, is not precisely known. That a tooth, whether it be designed for grinding hard or cutting soft sub stances, should do both the more effectually in the ratio of its firmer and more extended implantation, is intelligible. That a more perfect performance of a preliminary act of digestion should be a necessary correlation, or be in harmony, with a more complete conversion of the food into chyle and blood,— and that such more efficient type of the whole digestive ma chinery should be correlated, and necessarily so, with the hot blood, quick-beating heart and quick-breathing lungs, with the higher instincts, and more vigorous and varied acts of a Mam mal, as contrasted with a cold-blooded reptile or fish,—is also conceivable. To the extent to which such and the like reason ing may be true, or in the direction of the secret cause of the constant relations of many-rooted teeth discovered by observa tion,—to that extent will such relations ascend from the em pirical to the rational category of laws.
The interest which the above-described fossil from the Stonesfield oolitic slate excites is not exclusively due to its antiquity, its uniqueness, or its peculiarity ; much is attached to its relations as a test in palaeontology of the actual value of a single tooth in the determination of other parts of the organ ization of the animal. According to our opinion of these un seen parts, we frame our expression of the nature and affinities, or of the place in the zoological system, of the extinct species. From the resemblance of the lower molars of Stereopurthus to those of Pliolophus, which, though not close, is closer than to the teeth of any other known animal, it is probable that the Stereognathus was hoofed, and consequently herbivorous, or deriving the chief part of its subsistence from the vegetable kingdom. Cuvier has written,—" La premiere chose a faire dans l'etude d'un animal fossile est de reconnaitre la forme de ses dents molaires ; on determine par la s'il est carnivore ou herbivore, et dans ce dernier cas, on pent s'assurer, jusqu'a un certain point de l'ordre d'herbivores auquel it appartient." * In the case in question the form of the molar teeth of one jaw is recognizable, but the herbivority of the fossil is not thereby determined. We can only infer it to be more probable that the fossil was a Herbivore than an Insectivore or a mixed-feed ing Carnivore.
Admitting the herbivority of the fossil, it is not certain that it was hoofed ; there is nothing in the form and structure of the tooth to prove that. Both form and structure are compatible with the hoofless muticate type of herbivorous Mammal, as shown by the Manatee ; it is the small size of the Stereogna thus which renders it less probable that it was a diminutive kind of Manatee, and more probable that it was a diminutive form of Ungulate. But seeing the manifold diversities of the multi-cuspid form of molar teeth in recent and extinct insec tivorous unguiculate quadrupeds, it is not impossible but that the Stereognathus may have belonged to that order ; there is no known physiological law forbidding it.
The form of the cusps, and their regular symmetrical arrangement in the Stereognathus, as compared with the known modifications of multi-cuspid molars in certain small extinct forms of hoofed quadrupeds, constitute the grounds upon which an opinion is formed of its most probably belonging to the same section of Ungulata.
Then, is it not true, it may be asked, that by virtue of certain established laws of correlated structures, an extinct animal may be re-constructed from a single tooth or from a fragment of bone ? Is the Cuvierian basis, or what has been so regarded, of palaeontology unsound ? Not necessarily from aught that has been said or written on the subject of the Stereognathus. We do not know the comparative anatomy of the family of quadrupeds to which the Stereognathus belonged. What we do know of its teeth suggests that that family may have had modifications of the skeleton so far different from those of any, the modifications of which are known, as to have constituted a type of, perhaps, a marsupial family ; but a type as well marked, and as distinct, as the type of skeleton which Cuvier inductively studied in the feline Carnivora (fig. 128), and in the ruminant Herbivora (fig. 129), and by which pre liminary study he was enabled to enunciate that beautiful law of the " correlation of forms and structures" to which allusion has been already made, and which will be illustrated by exam ples, and its mode of application pointed out, in another part of the present work.
In certain instances of constant coincidences of structure, as demonstrated by comparative anatomy, the sufficient—z. recognizable, intelligible, or physiological—cause of them is not yet known. But, as Cuvier in reference to such instances truly remarks, " Since these relations are constant, there certainly must be a sufficient cause for them."* In certain other cases Cuvier believed that he could assign that "sufficient cause," and he selects, as such, the correlated structures in a feline Carnivore, and in a hoofed Herbivore. The physiological know ledge displayed by him in his explanation of the condition of those correlations is most exact ; its application in the restora tion of the Anoplotherium and Palceotherium most exemplary.