420 PROP. OWEN" ON AN AN03I0D0NT BEPTILE FEOM 



ridge (Ji) anconad of the ulnar border of the narrow part of the 

 humeral shaft. The entepicondyle (i) forms an angular prominence 

 terminating below the radial border of the shaft. 



The above characters of the humerus of the Platypodoscmrus have 

 been deemed worthy of detailing by reason of the Monotremes pre- 

 senting the nearest resemblance to such bone in the vertebrate series. 



In these alone (OrnitJiorhj/nchus, PL XYI. fig. 8, and Echidna, ib. 

 fig. 9) have I found such characters repeated, but even with some 

 exaggeration of the proportions of breadth to length. The crests 

 (/, c',), continued distad and palmad from the ento- (c) and ecto- 

 (6) tuberosities, leave a deeper intervening concavity at the proxi- 

 mal part of the palmar surface of the shaft, almost defining, as by a 

 low curved ridge-like border, such concavity in the Echidna (fig. 

 9, g). In both genera, also, besides the ridge extending distad from 

 the entotuberosity, c, there is a distinct angular tricipital ridge or 

 process {d) as in Platypodosaurus ; and this is most developed in 

 Echidna^ though with a minor relative extent of the base. 



The bridge {Ic) * is broader relatively than in Platypodosaurus ; 

 so much so in Platypus as to carry the entry of the entepicondylar 

 canal to the radial margin of that expansion of the distal end of the 

 humerus (fig. 8, o). The transverse development of the bridge is so 

 much greater in Echidna (fig. 9, h) that it assumes the character of 

 the palmar surface of the distal expansion of the shaft, and the entry 

 of the canal is pushed, as it were, quite to the anconal surface, and 

 the shaft thus seems to be perforated antero-posteriorly, but with 

 the normal obliquity (ib. o'). 



Cuvier notes the transference of the articulation (J) for the 

 antibrachium to the ulnar portion of the distal expanded end of the 

 humerus in Orniihorhynchus t, and also the proportion of the distal 

 breadth to the length of the humerus in that low gwasi-reptilian 

 type of mammal : both characters are exaggerated in Echidna (fig. 9). 



The attachment of the large " teres major " to the ridge c', and the 

 origin of a considerable pari: of the " triceps extensor cubiti " from 

 the process d, in the Monotremes J, have suggested the terms applied 

 to these ridges in the humerus of Platypodosaurus. 



Manus. — Of the digits cemented to the concave surface of the 

 scapula (PI. XVII. fig. 2), that next the humeral end (v) shows the un- 

 gual, 3, and middle, 2, phalanges, with part of the proximal phalanx, 

 on the hypothesis of the phalangial formula being the same as in fig. 5, 

 copied from the subject of plate lii. figs. 2 and 3 of my * Cata- 

 logue '§. 



* 111 a humerus of a large JDicynodon recently transmitted by Mr. Thos. 

 Bain, the ossification of the " bridge," which seems to have extended distad from 

 the fore part of the shaft along the primitive ligament, has not completed the 

 confluence with that part at the distal end of the bridge. 



t " La poulie articulaire qui est ici tout-a-fait globuleiise, se trouve loin d'etre 

 placee au milieu de I'os ; elle est au tiers externe." (Le9ons d'Anatomie Com- 

 paree, ed. 1835, tom. i. p. 386.) 



\ Meckel, ' Omithorhyncfd paradoxi Descriptio Anatomica,' fol. 1826; Owen, 

 Art. Monotrcmata, Cyclopaedia of Anatomy, vol. iii. 1841, p. 377, fig. 180. 



§ Op. cit. p. 54, pi. lii. 



