OF THE MESOZOIC MAMMALIA. 
261 
the first premolar and frequent atrophy, of the occasional prismatic elongation of 
the molars, {Ghrysochloris). 
All the above peculiarities are observed in different members of the Stylacodon- 
tidcB, some of which approach the typical insectivorous structure much more closely 
than others. The peculiar feature of some genera is the separation and pris- 
matic elevation of the crowns in a triangular section, as in Ghrysochloris, so that 
when the upper and lower series are brought together they form alternating trian- 
gles, with the bases turned outwards in the upper and inwards in the lower molars 
{AstJienodon) . The styliform elongation of the molar crowns is very extreme in some 
of the British Museum specimens of Stylacodon (No. 47,758) . The procumbent incisi- 
form canines and chisel-shaped incisors are seen in Owen’s type of Stylodon. The 
postero-internal cusp of Dryolestes is also found in Gahochloris. Altogether, while 
admitting the risk of systematic determination upon the basis of such analogies, we 
cannot deny that there is far more ground at present for placing the StylacodontidcB 
in or near the line of the Insectivora,^ than in any other order. 
No distinctive characters can be assigned at present to the Imectivora primitiva 
except the primitive formula, to distinguish them from the recent Insectivora, and the 
tritubercular molars, to distinguish them from the other J urassic groups. 
The above general phylogenetic conclusions may be summarized as follows : 
I. That the Jurassic members of the Second Group, although not forming a single 
distinct order, bear marks of comparatively recent origin from a common stem. 
II. They subdivide into at least two larger series, including six or more families, 
which respectively lead to the Marsupialia and to the Placentalia,, possibly to some of 
the existing families of the same. 
III. These series, as found at this period, have considerably diverged from each 
other and have assumed structural modifications which are at present peculiar to 
the Marsupialia and Insectivora. 
In addition to the acknowledgments made in the introduction, I wish in closing 
to express my indebtedness to Professor W. H. Plower, for his friendly aid extended 
to me while working in the British Museum ; to Professor S. F. Clarke, of Williams 
CoUege, for the loan of material ; to Professor O. C. Marsh, for the supply of cliches 
of his woodcuts. All the text illustrations, with the exception of these cuts, are the 
work of Mr. Rudolph Weber. I may call attention to the fact that since my first 
studies and sketches of the English types were made, I have been unable to re-examine 
^ Schlosser (op. cit., p. 137) leaves the phylogenetic position of the Chrysochloriida; in some doubt. Their 
limited distribution, unique dentition and anatomy, possibly denote that they represent a low persistent type. 
According to Peters, the mammary glands of the Cape mole {Chrysochloris) are without teats. Monats. 
Akad. Wiss. Berlin, 1865. 
