110 Dr. C. W. Andrews—The Skeleton of Peloneustes. 
A careful study of the mode of occurrence of elementary crystals in 
them shows that the growth of a skeleton augite is almost invariably 
accompanied by the elimination of iron oxide at intervals along the 
-stem and arms of the skeleton crystal. Here, the reverse occurs. 
The brown patch (which may be regarded as an enlarged basic 
elobulite) contains the ingredients both of the pyroxene and iron 
oxides, the latter apparently in excess of the former. The pyroxenic 
constituent separates out in globulitic forms, segregates at the edges 
of the patches, and is gradually eliminated as the globulitic processes 
break off and become definite elementary pyroxenes. It is not 
suggested that after this the brown patch merely assumes hexagonal 
outlines—although its somewhat marked angularity seems to indicate 
an inclination to do so. The process is, no doubt, much less simple, 
and must involve a differentiation between the hematite and the black 
oxide of iron; but it is certain that these minerals do assume a more 
or less definite crystalline form after concentrating as much as possible, 
and so eliminating the pyroxenic material. 
EXPLANATION OF PLATE XI. 
Rock-sEctTions, PRE-TrERTIARY Dyxr, Usway Burn. 
‘Fig. 1. This shows the general structure of the rock. There are two types of 
porphyritic felspars : the upper one belongs to the first set, and has few 
inclusions and clear sharp angles and edges. The lower one has rounded 
angles and is riddled with inclusions: it belongs to the second set. 
x 22°6 diameters. 
Shows the same field between crossed nicols. 
This shows the mode of occurrence of the elementary felspars. The 
‘hour-glass’ structure is well seen in the small longitudinal section 
which occurs near the centre of the field. x 260 diameters (approx.). 
., 4. This emphasizes the dark borders of the brown patches, and shows the 
globulitic nature of the processes. In the centre of the field there is 
a small hexagon of the micaceous iron oxide, in which different thick- 
nesses (due to incomplete cleavage-plates) have produced different 
intensities of colour. x 260 diameters (approx.). 
,, 5. An incomplete pyroxene is seen here. Although it occurs in a dark patch, 
it is surrounded by a zone of light-coloured material, which also fills the 
gap extending across the centre of the crystal. There are several small 
detached pieces of pyroxene near the lower left-hand corner of the 
crystal. x 208°3 diameters (approx.). 
,, 6. Shows the same with nicols crossed. This brings out clearly the isotropic 
nature of the colourless material which forms the zone and the central 
inclusion. 
cw bw 
2? 
IV.—Nors on a Movntep Sxererron or a Smart Prrosaur, PLLONEUSTES 
PHILARCHUS, SEELEY, SP. 
By C. W. Anprews, D.Sc., F.R.S., British Museum (Natural History). 
(PLATE XII.) 
Boa skeleton figured on Plate XII is that of a small Plosaur, 
Peloncustes philarchus, Seeley, sp. This specimen was obtained 
‘from. the Oxford Clay in the neighbourhood of Peterborough by 
Mr. A. N. Leeds, F.G.S., to whom the British Museum is indebted for 
-a great series of more or less perfect skeletons of many species of 
1 Published by permission of the Trustees of the British Museum. 
