SEA-SERPENTS 195 
discovered by Dr. Williston, complete in nearly every detail. The 
restoration of the skeleton of Platycarpus, shown in Plate XXX., 
is based chiefly on one specimen, but certain parts are supplied 
conjecturally from Tylosaurus. The latter has been restored, as 
to its skeleton, from three specimens found by Dr. Williston, 
who was fortunate enough to find an impression of the skin, 
near one of the paddles, showing small scales, rather like those 
of a snake, but these were too small to be shown in our 
restoration (Plate XXXI.) of Tylosaurus. Platycarpus seems to 
be intermediate between Clidastes and Tylosaurus. In Clidastes 
we see a very long body, and a shorter tail, but this tail must have 
Fic. 70.—Superior aspect of the cranium of Platycarpus curtirostris, Cope; from 
the Upper Cretaceous of N. America (greatly reduced). (After Cope.) 
been a powerful organ of propulsion. The limbs were strong 
and flexible, and the movements of this Mosasauroid reptile were 
more snake-like. The vertebre in the tail had their neural 
arches lengthened, and probably the tail had a fin-like expansion 
to help in propulsion, as in the case of eels. 
The accounts given by Professor Cope of his explorations and 
the difficulties encountered in procuring the valuable specimens 
on which his conclusions are based, are most interesting, and 
such as every fossil hunter will appreciate. We, in England, 
who visit clay pits, stone quarries, railway cuttings, etc., during 
a morning or afternoon walk, and return home at our leisure 
