626 S. W. WILLISTON 
The arrangement of the elements in Trimerorhachis will be 
shown in the figures better than I can describe them. The coronoid 
(cor), in its relations and position, is almost precisely like that of 
Labidosaurus, except that it is broader anteriorly, and does not 
extend quite as far forward. It bears the small teeth opposite 
the posterior dental series. The prearticular (pa) is a very broad 
bone, rather loosely united with the adjacent elements in most of 
the specimens. Its lower margin posteriorly forms the upper 
margin of the foramen for the chorda tympani. It borders, as 
usual, the upper inner margin of the posterior meckelian foramen 
m 
SET an 
AAAI A a; 
Fic. 1.—Right mandible of Trimerorhachis alleni (?) Case, four-ninths natural 
size: A, from within; B, from without. 
and then continues as a slender projection nearly or quite to the 
hind end of the splenial. The splenial (sp) has precisely the same 
relations anteriorly as in the cotylosaurs and pelycosaurs, entering 
more or less into the symphysis and extending back to the anterior 
margin of the anterior meckelian foramen, and beyond it above 
to a variable extent. These relations are confirmed in some 
twenty different mandibles. Lying back of this splenial there is 
another element corresponding to the posterior part of the 
splenial in Labidosaurus, except that it does not extend beyond 
the posterior meckelian foramen. It overlaps broadly above the 
anterior end of the prearticular. I propose to call this bone the 
