MERRIAM: TRIASSIC ICHTHYOSAURIA. 



33 



McGregor 1 " as the probable reason why the ribs of ichthyosaurs do not artic- 

 ulate on the upper arches. It is true that in the small forms with relatively 

 long and low centra, as in Delphinosaurus, the rib heads rest against the upper 



1> 



27 



\J 



28 



29 



Fi<*. 27. Deli>]iiin>nitHruss pcrrini Merriam. Lateral view of middle dorsal centrum, X %. 

 Fig. 28. Shastasaurus osmonti Merriam. Lateral view of middle dorsal centrum, X %. 

 Fig. 29. Shastasaurus careyi ( ?) Merriam. Lateral view of middle dorsal centrum, X %. 



arches to a considerable extent. It is also true, that in several species of 

 tasaurus, with vertebral centra as high and thin as in the typical Ichthyosaurus, 

 the rib heads of the cervical and anterior dorsal region are in contact with the 

 base of the neural arch. 



Neural Arches. The upper arches of the vertebrae in the Triassic genera 

 seem in every case to be thicker transversely than in the typical Ichthyosaurus. 



Fig. 30. Dclpltinosaimtx perrini Merriam. Middle dorsal vertebrae situated at the point in the series 

 where the diapophysial and neurapophysial surfaces of articulation are separated, X%. 



is McGregor, J. H., Mem. Am. Nat. Hist., vol. 9, pt. 2, p. 91. 



