Vol. 56.] ARISTODESMUS RUTIMEYERr. 637 



resemblance to Echidna is most striking in the femur and humerus, 

 it may be considered that the prolongation of the fibula proximally 

 so as to extend beyond the tibia is an approximation, as far as 

 it goes, to Monotreme character. The bones of the fore-leg are 

 capable of rotation, such as usually occurs in the fore-arm, the 

 fibula crossing the tibia obliquely. This probably indicates use of 

 the hind-limbs in burrowing. 



(a) The Femur. 



The femur is slender, compared with the massive femur of Pareia- 

 saurus, or even with the femur of Echidna. There is some appear- 

 ance of a twist in the bone, which is due to the slight transverse 

 expansion of the distal end. The affinities of the bone are closer 

 with Echidna than any known Anomodont, but the Anomodont 

 femur is less fully known than the humerus. The proximal end of 

 the bone has a smaller articular head than is seen in Echidna, and 

 although prominent so as to extend above the adjacent surfaces, is 

 less prominent than in Echidna, and may not reach quite to the 

 inner margin of the bone. The trochanter minor is prominent on 

 the inner margin, near the proximal end, and the ridge which it- 

 forms is prolonged down the shaft. It is unlike the corresponding- 

 ossification in Ornithorhi/nchus, Rhopalodon, Pareiasaurus, Tribol- 

 odon, or Cynognathus. The trochanter major is a thin external film 

 which widens the shaft. In Echidna it extends more than halfway 

 down the length of the femur; but while the trochanter is of exactly 

 the same type in this fossil, terminating in a straight external edge 

 which is thin, it is less expanded transversely, and less than half 

 as long as the bone. The vertical direction, of the head of the 

 bone upward in the fossil, rather favours comparison with the 

 Monotreme. There is no close correspondence with the Cynodont 

 type of femur, characterized by the singular transverse expansion 

 of the proximal end of the bone, and forward reflexion of the short 

 trochanter major, seen in Cynognathus and in Tribolodon. 



A similar restriction of the trochanter major to the proximal end 

 of the bone, and a longitudinal development of the trochanter 

 minor in Microgomphodon, distinguish that genus equally from 

 this fossil and from Echidna. Pareiasaurus is the Anomodont 

 that approaches nearest to this femur, which in its proximal end 

 is intermediate between Echidna and Pareiasaurus, and appreciably 

 nearer to the latter than to the former. It makes no approach in 

 the proximal end to the femur in Procolophon, which is massively 

 triangular, with a deep excavation behind the articular head on the 

 under side of the bone, between the small marginal major and 

 minor trochanters, being more intermediate between Pareiasaurus 

 and Ornithorhynchas. 



The middle of the shaft, on its superior aspect, is well rounded 

 from side to side. 



The femur appears to widen transversely at the distal end, while 

 its depth is augmented by the articulation with the fibula and 



Q. J. G. S. No. 223. 2 u 



