Apr. 1903. NorrH AMERICAN PLESIOSAURS—WILLISTON. 25 
been denied by Baur*. No indications of such an ossification have 
been found in adult reptiles, living or extinct, even in those in which 
the opisthotic remains as a permanently free ossification. The opis- 
thotic was previously called paroccipital by Owen in 1838, and the 
name must take precedence. Copet, however, suspected that the 
opisthotic or paroccipital is really composed of two elements, the 
outer of which is the true paroccipital, while the inner, that entering 
into the formation of the otic canals, may be properly called the opis- 
thotic. Baur denies this, insisting that there is but a single element, 
persistent in the Testudinata, Ichthyopterygia, the young of Sphenodon, 
and other Rhynchocephalha, as well as in some of the Cotylosauria ; 
firmly and indistinguishably fused with the exoccipital in all other 
reptiles, so far as is known; free, according to Cope, also, as the 
so-called squamosal of Baur, the paroccipital of Cope, the supratem- 
poral of Woodward}, in the lacertilia. 
If there be but one element here, and, so far, the evidence is 
inconclusive that there are two, then it must be called the paroccipital, 
a name first given to it by Owen. Andrews describes the element as 
distinct in the young of CryAtoclidus§, but there are no indications of 
it in the present specimen. 
The proétic of Huxley, the alisphenoid of Owen (Comparative 
*Zool. Anzeiger, No. 296, 1889; Journ. Morphology, 1889, p. 467. 
tT ‘‘ The opisthotic in reptiles is generally early fused with the exoccipital, but in the Ichthyop- 
terygia and TVestudinata it is distinct, and takes the place of the petrosal as asupport for the quadrate 
in conjunction with the exoccipital. In the Pythonomorpha a bone which occupies the position of 
the terminal part of the opisthotic (or paroccipital, which is the older name) issues from between 
the exoccipital and petrosal, and supports the quadrate. Whether this is homologous with part or 
all of the paroccipital is an open question. For the present I call it the paroccipital and it is 
probably a distinct element from the opisthotic.’’ Cope, Syllabus, 2d ed.,1898. A fuller description 
of the relations of this bone the reader may tind in my paper on the Mosasaurs (Univ. Kansas Geol. 
Surv., vol. iv, p. r21). After much reflection I believe that Cope is right in rejecting the term squa- 
mosal for this element, whatever it is. Parker describes and figures the opisthotic as a large ele- 
ment in the snake (1. c.), occupying its usuai and normal position. At the same time it is exceed- 
ingly difficult to believe that the remarkable relations of the bone in the mosasaurs can be those of 
the squamosal, occupying almost the normal position of the real opisthotic.. That the bone called 
the prosquamosal in the lizards is not the squamosal would also seem probable, though not 
impossible. I prefer to call the elements, until it be proven that there are two opisthotics 
in the lizard, the paroccipital and prosquamosal with Cope. It is of interest to note, however, that 
Cope, in his last edition of the Syllabus (published posthumously), retains the name of squamosal 
for the element he previously called the supratemporal (2. ¢., the prosquamosal). Further on he 
defines the plesiosaurs as follows: ‘‘ No supramastoid; paroccipital not distinct; a quadrato jugal 
arch; scapula triradiate; no clavicle; ribs one-headed.’’ Cope’s supramastoid.is the bone he thought 
erroneously to exist in the skull of Cimoliasaurus snow, that is the real squamosal if present, and 
Andrews assures us that it is sometimes present in the young animal. I donot understand what is 
meant by “no clavicle,’’ unless it be that he accepted Hulke’s determination of these elements as 
the omosternum, a subject which will be discussed further on. He forgets also that some plesio- 
saurs do have rudimentary double-headed ribs in the cervical region. 
Notwithstanding all! that has been written, the homologies of the temporal bars in the reptilia 
are yet uncertain, more so than any other parts of the reptilian skull. 
t Vertebrate Paleontology, 1898. 
§$ Geological-Magazine, 1895, p. 242. 
