THE REPTILIAN CLASS OF THE MOSASAURID^. 693 



The tympanic is a characteristic bone in the orders and minor 

 groups of Reptilia. In Crocodilia and Chelonia (fig. 1) it is wedged 

 between the mastoid, squamosal, and pterygoid. In Lacertilia (fig. 2) 

 the squamosal (27), instead of abutting upon the fore part of the 

 tympanic (.28), arches over it and combines with the mastoid (8) and 

 paroccipital (4) in its suspension. In Ophidia (fig. 3) the squamosal 

 is absent, and the tympanic (28) is suspended exclusively from the hind 

 end of a long and backwardly produced mastoid (figs. 4 and 8, 8 & 8', the 

 tympanic surface). The tympanic in Chelonia is remarkable for 

 its breadth in proportion to its length, and for the conspicuous de- 

 gree in which its function of supporting the " membrana tympani " 

 is shown, the entire, or nearly entire, circumference of the frame of 

 that membrane being contributed by the tympanic. 



In some Lacertilia (Thorictes draccena, Iguana, Twpinambis te- 

 gueocin, Scincus, and above all in Amblyrhynchus, fig. 19) the tym- 

 panic more or less resembles in shape that of the Chelonia in its 

 relative breadth and the proportion of the frame of the membrana 

 tympani which it exhibits ; but a gradation may be traced to the 

 more simple columnar shape, as in Varanus, Lacerta, Stellio, Gecko, 

 Chamceleo. 



In Ophidia, and especially in Python, the long diameter of the 

 tympanic is in excess, and the swing of the suspended column is 

 checked only by the abutment of the pterygoid, just above the arti- 

 cular surface for the mandible. Figures of the bone in Python Sebce 

 (9' & 10') are juxtaposed to those Mosasaurus Hoffmanni (figs. 9 k, 10). 

 In this species the tympanic is not only lacertian in type, but ex- 

 emplifies the chelonian proportions in a greater degree than in any 

 of the Lacertilia, in which order Amblyrhynchus comes nearest to 

 the gigantic Sea-Lizard in this respect. The breadth of the bone 

 in Ambl. cristatus, as in Mosasaurus, nearly equals the length ; it 

 frames, as in Mosasaurus, almost the entire circumference of the 

 drum-membrane, and forms the tympanic cavity, which is conical 

 and contracts from the basal frame to the otosteal orifice. 



In Python the tympanic column is trihedral, and the angle between 

 the fore and hind facets is turned outward as a thin plate ; the con- 

 trast between this bone and the broad outwardly directed base of 

 the subcircular hollow tympanic of Mosasaurus is extreme. 



The tympanic bone alone suffices to refute the ophidian hypo- 

 thesis of the Mosasauroids ; and only the obligations palaeontology 

 is under to the enlightened and liberal administration of the ' Geo- 

 logical Survey of the Territories of the United States,' under the 

 accomplished " Officer-in-Charge," F. V. Hayden, and the indefa- 

 tigable devotion of Dr. Cope to the discovery and collection of the 

 fossils figured in the fifty-seven plates of the richly illustrated 

 quarto volume issued in 1875 from the " Government Printing- 

 office, Washington," have imposed upon me the obligation to omit 

 no point which may bear upon the right interpretation of those 

 fossils. 



§ 5. Upper Surface of Face. — A significantly characteristic part 

 differentiating the skull of Lacertilia from that of the Crocodilia on 



Q. J. G. S. No. 132. 2z 



