202 Dr. C. W. Andrews—An Ichthyosaur from Peterborough. 
points of local wrenching of the tracks. As in the case of most 
earthquakes, the observations show abundant evidence that large 
blocks of the crust have been shifted laterally and vertically, with 
lesser adjustments of their component parts. When within the area 
covered by railway tracks, these adjustments may be measured by 
those displacements of the right-of-way which are indicated by lateral 
displacements between points of special wrenching. The Charleston 
earthquake, in the light of the established relationships, is brought 
into harmony with the greater number of large earthquakes which 
have been carefully studied in recent times. 
III.—Norrts on tHe Osreonogy oF OpHTHALMOSAURUS ICENICUS, 
SEELEY, AN IcnrHyosauRIAN Reprine FROM THE OXxFoRD CLAY OF 
PETERBOROUGH. 
By C. W. Anprews, D.S8c., F.R.S., British Museum (Natural History). 
URING the last few months the preparation of a descriptive 
catalogue of the magnificent collection of reptilian remains 
obtained by Mr. A. N. Leeds in the Oxford Clay of Peterborough, has 
rendered it necessary to examine in some detail the fine series of more 
or less complete skeletons of Ophthalmosaurus preserved in the British 
Museum. In the course of this examination a certain number of new 
facts have come to light, and since it will be some time before the 
detailed description can appear it seems desirable to give a brief 
account of some of the more interesting points. 
Comparison of a number of these skeletons with one another shows 
that an extraordinary degree of variability in the form of many of the 
bones occurred, the differences in the shape of certain elements, such 
as the quadrates or the basi-occipitals, beg so great in different 
skeletons that if these bones were found isolated they would almost 
certainly be referred to different species. Further examination 
shows that this would not be justifiable, since two skeletons of 
which the quadrates differ may have precisely similar basi-occipitals, 
or vice versd, the same thing holding good in the case of other parts. 
In fact, if any one part of the skeleton were taken as the standard of 
comparison, five or six species might be established which would not 
coincide with those that would be formed if another part were employed 
in diagnosis. ‘These variations are partly real and partly apparent. 
The real differences seem to be the result of the persistence of a large 
amount of cartilage in the skeleton of these marine reptiles, the 
consequence of this being that the actual form of the bones depends 
largely on the extent to which ossification had proceeded, and in any 
case is less definite than in land animals. The apparent differences 
are due to the pressure to which the bones have been subjected. 
This often leads to deformation without any actual fracture being 
visible, and is particularly noticeable in the case of the vertebre, 
the centra of which may be shortened by nearly half their length 
without any other distortion. For the above-mentioned reasons it 
has therefore seemed best to refer all the specimens to the species 
