THE PRIMITIVE STRUCTURE OF THE MANDIBLE IN 
AMPHIBIANS AND REPTILES 
S. W. WILLISTON 
University of Chicago 
There has been much uncertainty and doubt as to the structure 
of the mandible in the early amphibians and reptiles. The 
abundant material of paleozoic vertebrates in the University of 
Chicago museum has enabled me to determine beyond reason- 
able doubt, not only the intimate structure of the pelycosaurian 
and cotylosaurian mandibles, but also that of a very typical 
temnospondylous stegocephalian. A full discussion with figures of 
the various forms in which I have determined the structure will 
be published in a more extensive work on Permian vertebrates 
next June. For the present I give only the more essential, newly 
discovered characters in the cotylosaurian genus Labidosaurus and 
the stegocephalian Trimerorhachis, accompanied by figures of the 
latter. A brief description of the structure in Dimetrodon has 
already been published by me in Science. 
The coronoid of Labidosaurus extends from about the seventh 
tooth along the alveolar margin to about one inch back of the 
teeth, appearing on the inner side of the outer wall of the meckelian 
orifice as a narrow, thin bone. The bone forms the anterior wall 
of the orifice, joining the prearticular much as it joins the angular 
in the alligator. The prearticular covers the inner side of the 
mandible posteriorly much as in Trimerorhachis, though -not so 
broadly, and extends forward below the coronoid to about opposite 
the anterior end of the inframeckelian foramen, where it joins the 
splenial. The splenial, as usual, enters into the symphysis ante- 
riorly and extends back to the inframeckelian orifice about opposite 
the last tooth, and forms also part of the inferior margin of that 
foramen; it is only narrowly visible on the outer side of the 
mandible. 
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