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University of California Publications. [Geology 



The groups of reptiles which this form most closely resembles 

 in adaptation, so far as this is expressed in the structure of the 

 parts present, are the Placodonts and the Rhynchosaurs. Both 

 of these groups occur in the Trias, as does this form. The Placo- 

 donts were marine, and lived in approximately the same epoch as 

 Omphalosaurus ; the Rhynchosaurs were probably somewhat later 

 in appearance, and were not typical aquatic animals. Both types 

 differ from Omphalosaurus in the character of the mandibular 

 dentition and in the structure of the skull, so far as known. 



The jaw of Placodus possessed a large coronoid elevation. 

 The coronoid of the Rhynchosaurs is not so well known, though 

 such an element of considerable size seems to have been present in 

 II ijperodapedon minor. The region of the coronoid is not well 

 preserved in the Omphalosaurus specimen, and no definite state- 

 ment as to the presence or absence of the bone can be made. In 

 a form with crushing teeth like those seen here, such an elevation 

 is to be expected, and it may be that the crescentic element (c) 

 shown on plate 9 represents a discrete coronoid. 



The structure of the mandible does not agree with that of 

 either the Placodonts or the Rhynchosaurs. The character of the 

 palate, so far as is known, is quite different from that in both 

 groups. If the palatal elements are correctly interpreted, the 

 dentition cannot be compared with that of either Placodonts or 

 Rhynchosaurs, so far as occurrence of the teeth is concerned. 

 The mandibular dentition is nearer to Hyper odapedon than to 

 Placodus. 



Superficially this form has the appearance of a short-headed 

 Synapsidan. The head was evidently not greatly elongated and 

 the cranial region presumably relatively long. The structure of 

 the palate and of the mandible are not unlike that in the Plesio- 

 saurs. On the other hand, certain of the characters point toward 

 the rhynchocephalian type. The mandibular dentition is paral- 

 leled in the diaptosaurian groups, and if the two anterior ele- 

 ments («) are vomers, they probably correspond to the large pre- 

 vomers of the Diapsida. 



It is quite certain that Omphalosaurus represents a type far 

 enough removed from other known reptilia so that it must form 

 a distinct family, the Omphalosauridae, but the true position of 



