214 



KNOWLEDGE 



[NOVEMBEK 1, 1892. 



carried back at one bound very nearly to the commence- 

 ment of the secondary period. Subsequently, mammalian 

 remains were obtained from the trias of Somerset, which 

 proved to belong to the same genus as those from Stuttgart, 

 while others of a different type were found in the 

 equivalent deposits of North America. 



A little later, the year 18oi was made memorable by the 

 lirst discovery of mammalian remains in the freshwater 

 Pm-beck strata of Dorsetshire, belonging to the very top of 

 the Jurassic s)-stem : from which formation in subsequent 

 years a vast number of such remains were obtained, through 

 the energy of the Eev. P. B. Brodie and the late Mr. 

 Beccles. All these specimens were obtained from a single 

 bed, and many of them indicated forms more or less closely 

 aUied to those from Stonesfield and Stuttgart. It was 

 thus shown, once for all, that mammalian life must have 

 been locally abundant throughout the Jurassic period. 

 This conclusion was subsequently amplified by the 

 discovery in the upper Jurassic rocks of North America 



Fig. 3. 



-Lower jaw of Triconodon ; half natural size. 

 (After Marsli.) 



of a whole host of small mammals very closely allied to 

 those from Dorsetshire, a large number of which have been 

 described by Prof. 0. C. Marsh, some of whose figures are 

 here reproduced. Many of these small Jurassic mammals 

 (Figs. 2 and 3) were evidently carnivorous, and such 

 carnivorous forms exhibited two distinct types of dentition. 

 In one of them (Fig. 2) there was a numerous series of 

 cheek-teeth behind the tusk, or canine {a), each of which 

 carried three cusps arranged in a triangle ; while in the 

 other type (Fig. 3) the cheek-teeth were fewer in number, 

 and had the three cusps on then- crowns ranged in the same 

 line. From this peculiarity the animal to which the second 

 type of jaw belonged was appropriately named Tricmwdon. 

 The first type corresponds to the amphithere of the Stones- 

 field slate, while the second is more like the phascolothere 

 of the same formation. 



In our thu-d figure it will be observed that there is a 

 peculiar groove {y) rimning along the inside of the jaw, and 

 since a similar groove is found among existing mammals 

 only in the banded ant-eater and certain other carnivorous 

 marsupials, we have pretty conclusive evidence that 

 Triconodon and its allies were really marsupials. There 

 can also be but little doubt that the species of the amphi- 

 therian type ( I avoid mentioning the numerous genera of 

 these animals) are likewise members of the same order. 

 It is, however, quite possible that some of the Jurassic 

 mammals of Dorsetshire and North America may be more 

 nearly allied to that primitive group of mammals known 

 as Insectivores, among which are included the mole, the 

 shi'ew, and the hedgehog of Europe, as well as the more 

 generalized tenrec of Madagascar, and many other peculiar 

 creatures. All these insectivores are of a very low grade 

 of organization, and the result of modern researches is to 

 show that their connection with the marsupials is very 

 close indeed. Hence it is highly likely that some of the 

 Jurassic mamuials may have been the actual connecting 

 hnks between the marsupials and the insectivores ; and it 



-^-\ 



-Low'er ja\\ of PlafjiauJux: natural >i/A' 

 and enlarged. (After Marsh.) 



is worthy of mention here that while marsupials at the 

 present day linger on only in Australia and America, some 

 of the most primitive types of insectivores are preserved to 

 us in Madagascar, which is another refuge for animals of a 

 low grade of organization. 



There is yet another type of mammal found in the 

 English and 

 American ju- 

 rassics, to which 

 the Microh'stfs ..-— 



of the trias also \ 



appears to be- \ 



long, which has ^ 

 given rise to a 

 vast amount \ 

 of discussion 

 among palse- "■-■ 



ontologists. Fig. 4. 

 These remark- 

 able mammals 



are mostly of very minute size, and were long known only 

 by their lower jaws, of which a specimen is represented in 

 the accompanying figure : from which it will be seen at a 

 glance that the dentition is quite difi'erent from that of 

 either of the carnivorous types figured above. The lower 

 teeth comprise a single large incisor (t), behind which 

 were either three or four tall premolar teeth with cutting 

 edges, and marked on the sides with a number of oblique 

 grooves, from which the name Flti:iiinila.v was taken. 

 When unworn, these grooves extended along the whole 

 outer surface of the teeth, but when the teeth .have been 

 long in use (as in our figure) the groovings become worn 

 away from the sides. Behind these four premolars are 

 two smaller molar teeth, with the summits of their crowns 

 marked by a single longitudinal groove bounded by pro- 

 minent ridges. Now it was argued at first that this very 



peculiar type of 

 dentition mdica- 

 ted carnivorous 

 habit sin the 

 owners thereof ; 

 but it was sub- 

 sequently pointed 

 out that the ex- 

 isting rat-kanga- 

 roos of Australia 

 ( of which the 

 front of the skull 

 is shown in 

 Fig. 5) presented 

 a somewhat similar type of tooth-structure. Thus, the 

 last premolar tooth (pm.) of the rat -kangaroo has 

 a cutting - crown marked with a number of parallel 

 grooves ; while each half of the lower jaw terminates 

 in a single large incisor not unlike that of the Jurassic 

 Phviiiiula.v. Hence it was argued — and, in our opinion, 

 argued rightly — that as the living form is herbivorous, the 

 same must have been the case with the extinct one. When, 

 however, it was also urged that the rat-kangaroo and 

 Pla<iiinda.i: were closely allied animals, important dift'ereuces 

 between the two were overlooked. Thus, as will be 

 :ipparent from the figures, while in the former there was 

 but one grooved tooth, in which the grooves are vertical, in 

 the latter there were usually three or four such teeth in 

 ^\hich the grooves are oblique. Moreover, whereas the 

 recent form was provided with four molar teeth {m 1 — m 4), 

 the fossil had but two such teeth ; while the form of these 

 teeth was quite unlike in the two. Hence, when we add 

 that there are other important difi'erences between them 



Fig. 5. — Jaws and teeth of the Rat-Kangaroo. 



