Williston — Dinosaurian Genus Creoscmrus, Marsh. Ill 



Aet. VIII. — The Dinosaurian Genus Greosaurus, Marsh; 

 by S. W. Williston. 



The genus Allosaurus was proposed by Marsh in this 

 Journal for November, 1877, for the type species, A.fragilis, 

 from Colorado, presumably Canon City. His description was 

 as follows : " This genus may be distinguished from an} 7 known 

 dinosaurs by the vertebroe, which are peculiarly modified to 

 ensure lightness. Although apparently not pneumatic, they 

 have the weight of the centra greatly reduced by deep excava- 

 tions in their sides. Some of them have the centra hour-glass 

 in form, the middle part being so diminished as to greatly 

 reduce the strength. The vertebrse preserved are biconcave, 

 with shallow cavities. The feet bones referred to this species 

 are very slender. A lumbar vertebra has its centrum 105 mm 

 in length, and 80 ram in least transverse diameter. An anterior 

 caudal, 85 mm long, has its centrum so much constricted that its 

 least transverse diameter is 38 mm , while its anterior face is 90 mm 

 in transverse diameter." 



In the following March, Marsh described in this Journal the 

 genus Greosaurus, based chiefly, if not entirely, upon a left 

 ilium collected by the present writer at Como, Wyoming. It 

 will be observed that at the time of the erection of the genus, 

 very little was known of the distinctive characters of Allo- 

 saurus, and, so far as the author of it knew there was no 

 reason for referring the ilium and other bones there described 

 to another genus — or at least Marsh gave no reason. In the 

 following January number of this Journal, occurs the follow- 

 ing passage by Marsh, which, taken in connection with subse- 

 quent changes, is a little remarkable : 



" The genus Allosaurus is typical of the family [Allosauridce, 

 later merged into Megalosauridce] which also includes Creo- 

 saurus and Labrosaurus [Antrodemus Leidy]. The first 

 named genus presents some very interesting features in the 

 vertebrse and pelvic arch. The vertebrae first described are 

 remarkable for the reduction of the centrum by constriction, 

 so that the necessary lightness is secured without cavities in 

 the interior. This is shown in the lumbar vertebra repre- 

 sented in Plate X, figs. 3 and 4." 



From this it would certainly be inferred that the vertebra 

 figured was a type specimen, especially as it agrees in size and 

 form quite with the lumbar vertebra first described, and upon 

 which the genus and species Allosaurus fragilis practically 

 rests. In a later paper (this Journal, xvii, PL XIV), however, 



