THE OLDEST AIR-BREATHERS 28 1 



posterior limb was longer and stronger, and attached to a 

 pelvis so large and broad as to give the impression that the 

 creature enlarged considerably in size toward the posterior ex- 

 tremity of the body, and that it may have been in the habit of 

 sitting erect. The thigh bone is large and well formed, with a 

 distinct head and trochanter, and the lower extremity flattened 

 and moulded into two articulating surfaces for the tibia and 

 fibula, the fragments of which show that they were much 

 shorter. The toes of the hind feet have been seen only in 

 detached joints. They seem to have been thicker than those 

 of the fore foot. Detached vertebrae, which seem to be caudal, 

 have been found, and show that the tail was long and probably 

 not flattened. The limb bones are usually somewhat crushed 

 and flattened, especially at their articular extremities, and this 

 seems to have led to the error of supposing that this flattened 

 form was their normal condition ; there can be no doubt, how- 

 ever, that it is merely an effect of pressure. The limb bones 

 present in cross section a wall of dense bone with elongated bone- 

 cells, surrounding a cavity now filled with brown calcspar, and 

 originally occupied with cartilage or marrow. I desire to specify 

 the above points because I believe that most of the creatures 

 referred by Fritsch, Credner, and other European naturalists 

 to the Microsauria are of inferior grade to Hylonomus, though 

 admitted to present points of approximation to the true rep- 

 tiles. Woodward has recently described the remains of a 

 Microsaurian from the English coal formation. Nothing is 

 more remarkable in the skeleton of this creature than the con- 

 trast between the perfect and beautiful forms of its bones, and 

 their imperfectly ossified condition, a circumstance which raises 

 the question whether these specimens may not represent the 

 young of some reptile of larger size. 



The dermal covering of this animal is represented in part by 

 oval bony scales, which are so constantly associated with its 

 bones that I can have no doubt that they belonged to it, being, 



