306 W. W. Watts—On Outerops. 
possibility of this specimen belonging to the large form allied to 
P. plicatus (for which I propose to adopt Owen’s name P. truncatus), 
of which there are vertebrae from Ely in the British Museum. 
So far, therefore, as I can see, the forms described under the names 
of P. trochanterius, P. megadirus, P. brachistospondylus, P. Manseli, 
P. brachyspondylus (Phillips), and P. validus, belong to one and the 
same species. On the evidence of a detached pectoral girdle Prof. 
Seeley has, however, made P. megadirus the type of the genus 
Colymbosaurus, while P. Manseli is referred to a second genus, 
Murenosaurus, apparently on the evidence of the broken coracoids of 
the type specimen. I think it very probable, as already said, that 
the pectoral girdle in question does belong to the present species ; 
and I believe, moreover, that the pectoral girdle of the type specimen 
of P. Manseli when complete was (as Mr. Hulke states on p. 59 of 
the “ Proc. Geol. Soc.” for 1883) of precisely the same general form ; 
this form having apparently obtained in all the Upper and Middle 
Jurassic Plesiosaurs. 
As a climax to the treatment to which Plesiosaurs have been 
subjected we may notice Prof. Cope’s restoration of the so-called 
Hlasmosaurus platyurus, given in the ‘Trans. Amer. Phil. Soc.” vol. 
xiv. pt. i. pl. ii. In this instance the head has been placed at the 
extremity of the tail; and the Professor is consequently led to 
remark in his description that in the vertebra the prezygapophyses 
present the unheard-of peculiarity of looking downwards instead of 
upwards, while the so-called cervicals are indistinguishable from the 
caudals of other forms. 
Finally, after long consideration I have come to the conclusion 
that it will be convenient to separate from Plesiosaurus all those 
supra-Liassic species having single costal facets and a pectoral girdle 
without omosternum and the coracoids united by a median bar with 
the precoracoids. For these forms I propose to adopt the name 
Cimoliosaurus, Leidy, as being the earliest of the numerous terms 
which have been applied to this group. The typical forms have 
flattened terminal faces to the vertebree; but I do not propose to 
generically separate these forms like Plesiosaurus trochanterius and 
P. Oxoniensis in which these faces are cupped; although if such 
separation should be found advisable, I believe the term Polycotylus 
of Cope is the one which should be adopted. I shall show on 
another occasion that Hlasmosaurus of Cope is not separable from 
Cimoliosaurus. 
V.—Ovrtorops. 
By W. W. Warts, M.A., F.G.S., 
Fellow of Sidney College, Cambridge, and sometime Deputy-Professor of 
Geology at Oxford. 
OW that mapping constitutes such an essential part of field-work, 
it may be of use to some of your readers to connect together 
a few rules which have occurred to me on this subject. 
Valley-Outerops.—Professor Green has devised an admirably com- 
mon sense method by which the outcrop of a flat rock-bed can be 
