O. C. Marsh—New Characters of Mosasauroid Reptiles. 85 
skullof Zylosaurus micromus Marsh, and one of the specimens is 
represented i in the figures below, which give its main characters. 
e upper end is obliquely truncated, and occupied by a rugose, 
concave face (a) resembling an imperfect suture. The shaft is 
slender, and somewhat curved. The lower end is expanded, and 
has two distinct facets on the outer and inner angles (b and e), 
where it was attached to other bones. In Lestosaurus there are 
byoid bones very similar in form to those here described. 
Figure ae ry views of a hyoid bone of Tylosawrus micromus, Marsh; one-half 
na 
bier. and Tylosaurus, there is 28 arently — pair 
of hyped bones, more slender than the one here figured 
Tar Screrotic Priares. 
In the genera Lestosaurus and Tylosaurus, va orbit was pro- 
tect by. a ring of osseous plates, somewhat like those in 
Ichthyosaurus, said a few recent birds. This oe was composed 
of only a single row of plates, which in position overlapped 
than eit and their general features are shown in re figures 
, leening when united a regular imbricated ring. Besides this. 
Variety, however, two other kinds are found in Momsente 
These three kinds oceur in the same orbit. All of these ens 
are somewhat curved, with the concavity external. The outer — 
margins are thickened, and the inner thin and sharp. The 
