266 



PROP. OWEN ON THE SKELETON OF 



20. Description of Parts of the Skeleton of an Anomodont Eeptile 

 (Platypodosatjrusrobustus, Owen). Part. II. The Pelyis. By 

 Prof. Owen, C.B., F.G.S., &c. (Head March 9, 1881.) 



[Plate X.] 



The pelvis of Platypodoscmrus, which has heen relieved from the 

 matrix since the communication of the former paper*, includes five 

 sacral vertebra? (PI. X. fig. 1, 1-5), the right " os innominatum," 

 62-64, and a large proportion of the iliac constituent of that of the 

 left side. Of the first, or foremost, sacral vertebra, s \ the part of 

 the centrum in advauce of the transverse processes, d l , is broken 

 away, but so as to show the apex of the anterior conical articular 

 cavity. The lumbar vertebra, which was articulated therewith, but 

 had become dislocated therefrom, is preserved in the contiguous 

 matrix ; and the shape and depth of the articular cavity of its centrum 

 are exposed (PI. X. fig. 5). 



The transverse process of the first sacral, cl 1, including both di- and 

 pleurapophysial elements, is short and massive : measuring one inch 

 and a half in antero-posterior diameter at its middle and narrowest 

 part, it rapidly expands to its articulation (and what appears to be its 

 confluence) with the ilium, 62. Behind the transverse process the cen- 

 trum is moderately constricted, but expands to form the joint with 

 the second sacral, of which joint the transverse diameter is 2 inehes. 

 The breadth of the entire vertebra is 4^ inches. 



The centrum of this second sacral vertebra (s 2) shows a greater de- 

 gree of constriction and a narrower inferior or ventral surface. The 

 transverse process (ib. fig. 1, cl 2) extends from the anterior two thirds 

 of the upper part of the side of the centrum, is narrower than that 

 of the first sacral, but is longer, curving outward and backward, ex- 

 panded at both ends, but most so where it abuts against the ilium. 

 The breadth of the second sacral is 5 inches. The line of confluence 

 with and abutment against the ilium is better marked than in the 

 preceding vertebra. 



The centrums of the third and fourth sacrals are less expanded 

 at their mutual junction than are those of the first and second 

 vertebras. The transverse processes are rather shorter, as in that 

 of the third sacral, and this is narrower than those of the second 

 and fourth vertebras ; that of the fifth sacral is the narrowest. 



Each transverse process of the sacrum expands at its outer end 

 so as to touch the contiguous one where it joins or coalesces with 

 the ilium. The foramina or vacuities so circumscribed decrease in 

 size from the foremost to the third pair, and change the oblong for 

 the subcircular figure. The hinder articular end of the fifth sacral 

 centrum, ss, is broken away, but in a minor degree than the fore 



* Quart. Journ. Geol. Soc. for August 1880, vol. xxxyi. p. 414," plates xvi. & 

 xvii. 



