422 TROF. OAVEX OlSr AI^ Al^OMOJDONT REPTILE PROM 



Of Platypodosaurus the proximal portion of the right femur 

 (PI. XVII. figs. 6 & 7) was successfully extricated from the matrix ; 

 it is 7 inches in length, 2 inches across the longer diameter of 

 the shaft at the fractured end. But what proportion of the length 

 of the bone is preserved cannot be determined from the specimen ; 

 it seems to include at least one half of the femur. So far as the 

 shaft is preserved, there is a feeble trace of a small or third tro- 

 chanter close to the inner border. The outer or great trochanter (ib. 

 5, V) is remarkable for its extent longitudinally, which measures 

 5 inches and terminates abruptly at almost a right angle with the 

 shaft. The great trochanter is relatively shorter, and more gradually 

 bubsides upon the shaft, in Pareiasaurus (ib. fig. 8, 5, h'). From 

 tile upper part of the head of the femur (a) what answers in Pla- 

 typodosaurus to the neck (fig. 6, c) extends outward more hori- 

 zontally than in Pareiasaurus (fig. 8, c), but equally without any 

 concavity or depression, and gradually thickens from 9 lines across 

 near the head to 1 inch at the proximal end of the great trochanter. 

 The anterior surface between the head and great trochanter is 

 slightly concave through the prominence of these parts ; and there is 

 no eminence dividing that part of the anterior surface of the femur 

 "into two facets," as in Pareiasaurus^ . The posterior surface of the 

 proximal portion of the femur of Platypodosaurus shows a pair of low 

 broad ridges dividing such surface into three subequal longitudinal 

 tracts, the two outer ones of which are slightly concave across. There 

 is a submarginal trace of a "small trochanter;" but such process, 

 in the femur of Pareiasaurus (ib. fig. 8,/), projects like a tube- 

 rosity from the middle of that surface. A transverse polished 

 section of the fractured end of the femoral shaft of Platypodosaurus 

 (ib. fig. 6 a) exhibits a well-defined medullary cavity of corre- 

 sponding elliptical form ; but it is not more than 8 millims. in long 

 diameter, that of the section exceeding 2 inches. In Pareiasaurus 

 the peripheral portion of the femoral wall, for an extent of about one 

 seventh of the long diameter of the section of the shaft (ib. fig. 8 a), 

 presents a compact structure ; but this somewhat suddenly degene- 

 rates into a looser spongy texture, the large cells or interspaces 

 of which are occupied by a matrix susceptible of polish. In one 

 portion of a femur of Par. homhidens the looser osseous tissue had 

 so far decomposed that the invading matrix simulated the appear- 

 ance of a large medullary cavity ; but this showed instructive 

 evidence here and there of osseous tissue, and presented no 

 boundary such as is shown in Platypodosaurus, where the true 

 narrow cylinder is as well defined as in Ornitliorynclius and 

 Eclddna (fig. 9 a), though of less relative size. 



The fossil femur on which the reptilian genus Euslcelesaurus, 

 Huxley, is founded is stated to show " a large and distinct 

 medullary cavity "f. 



The chief characteristic of the femur of Platypodosaurus is the 



* Loc. cit. specimen No. S. A. 28. 



t Quarterly Journal of the Geological Society, vol. xsxiii. (1866) p. 2. 



