Apr. 1903. North American Plesiosaurs — YVilliston. 25 



been denied by Baur*. No indications of such an ossification have 

 bet D 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 Rhynchocephalia, 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 Crypt oclidns^i but there are no indications of 

 it in the present specimen. 



The proStic of Huxley, the alisphenoid of Owen (Comparative 



*Zool. Anzeiger, No, 298, 1889; Journ. Morphology, 1S89. p. 467. 



t " The opisthotic in reptiles is general! 3 early fused with the exocciphal, but in the Ichthyop- 



ter\yia ami Testudinata it is distinct, and takes the place of the petrosal as a support for the quadrate 

 in conjunction with the exoccipital. In the Pythonomorpha a hone which occupies the position of 

 the terminal part of the opisthotic (or paroccipital. which is the older namei issues from between 

 the exoccipital and petrosal, and sifpports the quadrate. Whether this is homologous with part or 

 all of the paroccipital is an open question. For the present 1 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 hone the reader may find in my paper on the Mosasaurs il'niv. Kansas Geo), 

 Surv.. vol. iv. p. 121 1. After much reflection I believe that Cope is right in rejecting the term squa- 

 mosa! for this element, whatever it is. Parker describes and figures the opisthotic as a larire ele- 

 ment in the snake (1. C), Occupying its usuai and norma! position. At the same time it is exceed 

 ingly difficult to heiieve that the remarkable relations of the hone in the mosasaurs can he those of 

 tmosal, occupying almost the normal position of tile real opisthotic. That the bone called 

 squamosal in the lizards is not the squamosal would also seem probable, though not 

 impossible. 1 prefer to call the elements, until it he proven that there are two opisthotics 

 in the li/ard, the paroccipital and prosquamosal with Cope. It is of interest to note, however, that 

 Cope, in hi- last edition of the Syllabus 1 published posthumously), retains the name of squamosal 

 for the element he previously called the supratemporal 1 /'. r.. the prosquamosali. Further on he 

 defines the plesiosaurs as follow.-: " No supramastoid; paroccipital not distinct; a quadralo jttgal 

 arch: scapula triradiate; no clavicle: ribs one-headed." Cope's supramastoid is the hone he thought 

 erroneous! 3 to exist in the skull ol Cimoliasaurus snowii, that is the real squamosal if present, and 

 Andrews assures us that it is sometimes present in the young animal. 1 do not understand what is 

 meant by " no clavicle." unless it he that he accepted Hulke's determination of these elements as 

 the omosternum, a subject which will he 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 set uncertain, more so than any other parts of the reptilian skull. 



% Vertebrate Paleontology. 1898. 

 g Geological Magazine, 189;. p. 242. 



