American Museum of Natural History

462 OSBORN, THE REPTILIAN SUBCLASSES DIAPSIDA AND SYNAPSIDA.
formed from the united trabeculae"; this is homologous with the large median
membrane bone in the Amphibia, named "parasphenoid" by Huxley, which,
according to Parker, " first appears in the frog as a parostosis beneath the intertrabecular
space." In the Dicynodonts this median or true vomer is greatly
developed and in the Theriodonts it is also strongly developed as a support of
the secondary palate. In the Chelonia, the median vomer is also developed as
the support of the palate, a condition somewhat resembling that of the Dicynodonts.
In the Plesiosauria, however, the vomer (parasphenoid) is reduced. Thus
in all the specialized Synapsida except the Plesiosauria a large true vomer is
present. In the known ancestral Synapsida or Cotylosauria, however, the vomer
is characterized by Broom as small.
Prevomers.-The bones in the Lizard and Sphenodon, which have usually
been regarded as homologous with the mammalian vomers, are really entirely
distinct paired elements, which are formed in connection with the nasal capsules
and are termed by Broom " prevomers." These are well represented in the
palate of the following Diapsidan types and orders: Rhynchocephalia, Pelycosauria,
Ichthyosauria, Dinosauria. They are also large in the Cotylosauria, and
in the supposed Synapsidan Plesiosauria. In all other specialized Synapsidan
types, namely, Anomodontia, Theriodontia, Testudinata, Mammalia, the prevomers
are small, vestigial, or wanting. In the Monotremata the prevomers are
represented by the dumbbell bones of Ornihkorhynchus. In the Marsupialia
they are replaced by backward projections of the premaxillze. In the higher
Mammalia they are probably present in the embryos of Edentata and Chiroptera
(Miniop/erus).
In general, therefore, if Broom is correct, it may be said that a large median
vomer is characteristic of the Synapsida (Chelonia, Anomodontia, Mammalia),
with the exception of the Plesiosauria, in which this element is present or
-reduced, and of the known Cotylosauria, in which it is small. The prevomers
are small in most Synapsida, but are large in the Cotylosauria, and present in the
Plesiosauria. The prevomers, on the other hand, are specially large and characteristic
of the Diapsidan orders; whereas the vomers are small or vestigial in
these orders.
Hypocen/ra.
In Diapsida the compound or separate condition of the atlas and axis, the
presence of hypocentra throughout the entire vertebral column or in a number
of the cervicals and caudals is a characteristic condition. In the Testudinata and
Plesiosauria the anterior cervicals are less distinctly temnospondylous, but it is
clear that the Mammalia, in which free hypocentra occur, are derived from Anomodontia
and Cotylosauria in which these elements are persistent.
Coracoid and Procoracoid.
This seems to be one of the most characteristic of all the lines of division.
In the Diapsida (Fig. 6) only comparatively few forms (certain Pelycosauria, Fig.