The largest pre-molar (see Fig. 305) in the fossil genus exhibits in like manner seven parallel grooves, producing by their termination a similar serrated edge in the crown; but their direction is diagonal—a distinction, says Dr. Falconer, which is “trivial, not typical.” As these oblique furrows form so marked a character of the majority of the teeth, Dr. Falconer gave to the fossil the generic name of _ Plagiaulax._ The shape and relative size of the incisor, _ a,_ Fig. 306, exhibit a no less striking similarity to Hypsiprymnus. Nevertheless, the more sudden upward curve of this incisor, as well as other characters of the jaw, indicate a great deviation in the form of Plagiaulax from that of the living kangaroo-rats. Fig. 304: Pre-molar of the recent Australian Hypsiprymnus Gaimardi, showing 7 grooves at right angles to the length of the jaw. Fig. 305: Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii, showing 7 diagonal grooves. Fig. 306: Plagiaulax Becklessi. Right ramus of lower jaw. There are two fossil specimens of lower jaws of this genus evidently referable to two distinct species extremely unequal in size and otherwise distinguishable. The _Plagiaulax Becklesii_ (Fig. 306) was about as big as the English squirrel or the flying phalanger of Australia (_Petaurus Australis,_ Waterhouse). The smaller fossil, having only half the linear dimensions of the other, was probably only one-twelfth of its bulk. It is of peculiar geological interest, because, as shown by Dr. Falconer, its two back molars bear a decided resemblance to those of the Triassic _ Microlestes_ (Fig. 389), the most ancient of known mammalia, of which an account will be given in Chapter XXI. Up to 1857 all the mammalian remains discovered in secondary rocks had consisted solely of single branches of the lower jaw, but in that year Mr. Beckles obtained the upper portion of a skull, and on the same slab the lower jaw of another quadruped with eight molars, a large canine, and a broad and thick incisor. It has been named Triconodon from its bicuspid teeth, and is supposed to have been a small insectivorous marsupial, about the size of a hedgehog. Other jaws have since been found indicating a larger species of the same genus. Professor Owen has proposed the name of _Galestes_ for the largest of the mammalia discovered in 1858 in Purbeck, equalling the polecat (_Mustela putorius_) in size. It is supposed to have been predaceous and marsupial.