AND ON THE EVOLUTION OF THE CROCODILIA, 435 



Lacertilian ; and, except for the great vacuity of the inner wall of 

 the acetabulum, it is almost intermediate in form between the ilium 

 of JStagonolepis and that of a Monitor or Iguana. 



From the proportion of the ilium to the centra of the vertebrae, it 

 is impossible that more than two vertebras could have entered into 

 the sacrum, which is again a point of approximation to the Lacertilia 

 and Crocodilia. 



The bone described and figured * by Riley and Stutchbury as an 

 ischium is really a humerus, as I have satisfied myself by having the 

 matrix which obscured the distal articular end of the bone cleared 

 away. In the collection of the Bristol Museum there are two of 

 these bones, both belonging to the right side. They are altogether 

 similar in their salient characters to the humeri of a Monitor. 



Another important point bearing on the affinities of the Theco- 

 dontosauria is the structure of the distal end of the tibia. I have 

 shown t that, in the typical Dinosauria, the distal half of the bone is 

 flattened, and has one face turned forwards and outwards, and the 

 other backwards and inwards, as in birds. Unfortunately I have 

 been able to find no tibia of Thecodontosaurus with the distal end 

 perfect. ; but, in two specimens, enough is left to show that its distal 

 extremity was not flattened, but was thick, with an almost circular 

 transverse section. In fact, in this respect it resembles the tibia of 

 a Lizard rather than that of a Dinosaurian. This structure of the 

 distal end of the tibia necessarily involves the absence of the 

 characteristic Ornithoscelidan astragalus. I may add that, in its form, 

 and its approximation to the proximal end of the bone, the inner 

 trochanter of the femur in Thecodontosaurus nearly resembles the 

 same part in the Monitor, while in the great length of the bones of 

 the limbs as compared with the vertebras there is a further approxi- 

 mation to many Lacertian forms. 



The measurements of certain bones found closely associated toge- 

 ther enable one to form a tolerably clear notion of the proportions of 

 Thecodontosaurus. A right humerus, 6*2 inches long, is associated 

 with the proximal halves of a right femur and a left tibia. Complete 

 specimens of femora and tibiae of the same size as the foregoing, 

 and lying close together, are respectively rather less than 7 inches 

 and 7'1 inches in length. A fragment of a tibia of approximately 

 the same size is associated with two ilia (right and left) and a 

 centrum of a caudal vertebra. The ala of the ilium measures 

 4 inches antero-posteriorly. The greatest antero-posterior dia- 

 meter of the acetabular portion is 2-7 inches. The centrum of the 

 vertebra is 1*2 inch long, 0*6 inch in vertical height. 



A strong curved ungual phalanx, 1*15 inches long, lies beside 

 a caudal vertebra of the same length as the foregoing ; and a similar 

 phalanx adheres to a metacarpal or metatarsal bone, 3-5 inches long. 



Thus it is clear that the Thecodontosauria possessed remarkably 

 elongated and, at the same time, strong limbs, differing from those 



* Trans. Geol. Soc. v. pi. 30, fig. 4. 



t " Further evidence of the affinities between Dinosaurian Keptiles and 

 Birds," Quart, Journ. Geol. Soc. vol. xxv. (1869) pp. 19-22. 



