AIR-BREATHERS OF THE COAL PERIOD. 169 



•of the lower jaw was about forty, and the number in each max- 

 illary bone about thirty. The teeth are perfectly simple, hollow 

 within, and with very fine radiating tubes of ivory. (Fig. 1, a and 

 b.) The vertebras have the bodies cylindrical or hour-glass 

 shaped, covered with a thin, hard, bony plate, and having within 

 a cavity of the form of two cones, attached by the apices. This 

 cavity was completely surrounded by bone, as it is filled with 

 stained calc-spar in the same manner as the cavities of the limb 

 bones. It was probably occupied by cartilage. The vertebrae were 

 ■apparently bi-concave. The neural spines are short and broad, with 

 zygapophyses, and are not separable from the bodies, the 

 neural arches being perfectly anchylosed to the bodies of the ver- 

 tebras. There are, on the dorsal vertebrae, strong diapophyses or lateral 

 spines, to which the ribs were articulated. (Figs. 15, 16, 17.) The 

 ribs are long, curved, and at the proximal end have a shoulder and 

 neck. (Figs. 1, 10, 18.) They are hollow, with thin hard 

 bony walls. The anterior limb, judging from the fragments pro- 

 cured, seems to have been slender, with long toes, four or possibly 

 five in number. A humerus is seen in fig. 1, and bones of the 

 toes magnified in fig. 11. The 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 extremity of the body, and that it may have been in the 

 habit of sitting erect. The thigh bone is well formed, with a dis- 

 tinct 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, but the 

 length of the tail is unknown. The limb bones are usually some- 

 what 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, 

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

 present in cross section awall of dense bone with elongated bone- 

 cells, surrounding a cavity now filled with brown calc-spar, and 

 originally occupied with cartilage or marrow. (Figs. 12, 13, 14.) 

 Nothing is more remarkable in the skeleton of this creature than 

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

 and their imperfectly ossified condition, a circumstance which 



