FOSSIL RODENTS. 



155 



SIDE VIEW OF SKULL AXD LOWER JAW OF MESOTHEIU'JM CRISTATUK. 



is in the Pliocene, three species having been described from deposits of that age at CEningen and 

 Montpellier. The family is, however, carried further back in time by the genus Titanomys, in which 

 the molars differ but slightly in structure from those of Lagomys, but there are only four of them in 

 each series, both above and below. 

 Two species of this genus have been 

 lecorded from Miocene deposits in 

 France and Germany. 



" 



We have thus passed very briefly 



in review the fossil Rodents belonging 



to the two great sections of the order 



to which all its living species are to 



be referred ; and it will be seen that 



while a knowledge of their existence 



is necessary to complete the history of 



the order, they present none of those 



peculiar characters which lend such 



interest to the fossil members of 



many other orders. There is, however, 



one fossil South American type to 



which we have yet to refer, as, by the 



curious combination of characters which 



it presents, it has long been somewhat 



of a puzzle to palaeontologists, and 



although generally placed among the 



Rodents, its peculiarities are such 



that Mr. Alston found himself com- 

 pelled to establish a third primary section of the order for its reception. According to M. Bravard, 



the first discoverer of this peculiar type, the Pliocene deposits of the Pampas of La Plata contain 



the remains of three species belonging to it; but the bones which have been sent to Europe, and 



which represent most parts of the skeleton, seem all to 

 belong to a single species, which has been very fully 

 described by M. Serres under the name of Mesotherium 

 cristattim.* What distinguishes it at once from all 

 other known Rodents is the presence in the lower jaw of 

 four incisor teeth, the second pair being very small and 

 placed immediate!} 7 behind the outer edge of the broad 

 middle pair. The latter are peculiarly widened and com- 

 pressed from front to back in both jaws, and their 

 summits, instead of being worn to a sharp chisel-like 

 edge as in ordinary Rodents, show an elongated ring of 

 enamel surrounding a slightly depressed surface. Hence 

 Mr. Alston denominated this section HEBETIDENTATA, 

 or BLUNT-TOOTHED RODENTS. The enamel in all the 

 incisors is continuous round the tooth. The molar teeth 

 are rootless and curved, the convex side being directed 

 outwards, contrary to what occurs in other Rodents. 

 They are surrounded by enamel, and show re-entering 

 folds which differ in the two jaws. Their number on 

 each side is five in the upper and four in the lower 

 jaw. The skull is massive, with enormously-developed 



sagittal and occipital crests, the latter of which run forward so far as to join the zygomatic arches; and 



these crests rise so high that the upper surface of the actual brain-case is entirely concealed by them 

 * Described almost at the same time by M. Bravard under the name of Typotherium. We here employ M. Serres' name. 



DENTITIOX OF MESOTHERIUM CRISTATUM. 



(A) Upper Jaw ; (B) Lower Jaw ; (c) Incisors. 



