MESOZOIC MAMMALS III 
by Marsh as parts of a coracoid and interclavicle. The peculiar 
character of the whole dentition of these forms indicates that if 
they are really Prototherians they cannot be regarded as primitive 
and ancestral types. 
It would be beyond the scope of the present work to describe 
in detail, or even to mention the names of all the members of 
this group, and it will therefore suffice to refer to a few of the 
principal types. Of the forms with tubercular premolars the best 
known is the genus Tritylodon (Fig. 24), which occurs typically 
in beds of Lower Mesozoic in South Africa, but is also known from 
the Trias of Stuttgart. In the Stonesfield Slate, near Oxford, 
which belongs to the lower part of the Jurassic system, and is 
separated from the Trias by the intervening Lias, a fragmentary jaw 
with three teeth (Fig. 27) appears to indicate an allied type, the 
teeth having three longitudinal ridges separated by grooves. In 
the Purbeck beds of Dorsetshire, forming the top of the Jurassic 
system, we find another member of this group, which has been 
described as Bolodon, closely allied to which is <Allodon of the 
Upper Jurassic of the United States. 
The first discovery of the remains of Mesozoic mammals was 
made in the Keuper or Upper Trias of the Rhetian Alps in 
Bavaria. In 1847 Professor Pleininger of Stuttgart, while sifting 
some sand from the Keuper of Diegerloch and Steinenbronn, 
found, among an immense mass of teeth, scales, and unrecog- 
nisable fragments of skeletons of fish and saurians, two minute 
teeth, each with well-defined, enamelled, tuberculated crowns 
and distinct roots, plainly showing their mammalian character. 
These were considered by their discoverer to indicate a predaceous 
and carnivorous animal of very small size, to which he gave the name 
of Jicrolestes antiquus. Subsequently Mr. C. Moore discovered in a 
bone bed of Rhetie (topmost Trias) age, filling a fissure in the 
Mountain Limestone at Holwell, near Frome in Somersetshire, 
various isolated teeth with their crowns much worn, but apparently 
including both upper and lower molars and a canine, which are 
assigned by Sir R. Owen to Pleininger’s genus JJicrolestes, and 
described specifically as Jf mocrei. Under the name of Aypsi- 
prymnopsis rheticus, Professor Boyd Dawkins described a single tooth 
with two roots discovered in the Rhetic Marlstone at Watchet in 
Somersetshire. Sir R. Owen referred the latter tooth to Vicrolestes, 
and if its describer is right in regarding it as a much worn premolar 
of the type of those of Plagiaulax (Fig. 25) there would be evidence 
that Microlestes was closely allied to the latter, from the molars 
of which those of J/icrolestes are scarcely distinguishable. 
Plagiaulax, of the Dorsetshire Purbeck (Figs. 24, 25), is at once 
distinguished from Tritylodon by its secant premolars, which, as already 
mentioned, recall those of some of the J/wcrepodide, although readily 
