92 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



Humerus. — The head is large, oval, anteroposterior diameter slightly- 

 greater than transverse diameter. The trochanters are not preserved. 

 The shaft is slender, deltoid crest present but weak. The entepicon- 

 dyle, slightly broken, is large, and the foramen present but small. The 

 external side of the distal end is broken, and the ectepicondylar region 

 is not preserved. What remains of the radial articulation suggests 

 that it was nearly spherical. The trochlea is not a broad groove 

 bounded by a sharp crest as in later mammals (except monotremes, 

 in which, however, these articulations are still less Ptilodus-Y^ke, than 

 are those of primitive Theria) but forms another subspherical, slightly 

 crested, condyle. The fossa for the olecranon is sharp and deep. 



Ulna. — The proximal end of the ulna has a nearly round, concave 

 articular surface. The shaft is stout, with a strong biceps insertion. 

 The distal end, which lacks the epiphysis, is widely expanded. 



Pelvis. — Broom (1914) interpreted what Gidley took for the pelvis 

 as a shoulder girdle and restored it in such a way as closely to resem- 

 ble the monotreme shoulder girdle, but Granger added a note to 

 Broom's paper (at Broom's request) pointing out that Gidley's opinion 

 was correct. This was based on the fine Eucosmodon material later 

 described by Granger and Simpson (1929) and by Simpson and Elft- 

 man (1928). There can be no doubt that the element is a pelvis, 

 although it is so extraordinary that Broom's error was quite under- 

 standable.^ The detailed descriptions of the Eucosmodon jjelvis 

 already published make it unnecessary to go into any detail regarding 

 that of Ptilodus. k.^ far as one can judge from the imperfect material, 

 the two are closely similar throughout, except that in Ptilodus the 

 pelvis is slenderer, with weaker muscle origins. 



Femur. — The femur of Ptilodus is also much like that of Eucosmodon, 

 except for being smaller and relatively weaker. The great trochanter 

 does not rise so far above the head, and the lesser trochanter is rela- 

 tively a little smaller, with less expanded head. 



Tihia andjibula. — These bones are poorly preserved and show Httle 

 except their relative sizes and the remarkably deep posteroproximal 

 excavation of the shaft, as in Eucosmodon. 



Measurements of the two types included here and the most important 

 statistical data follow. Here, and elsewhere, I do not give all the 

 many measurements on which these figures depend. The data are 

 given in a form that shows all the essential and few or no nonessential 

 figures and that makes comparison much easier and more reliable than 

 the publication of long tables of raw data, 



" In Gidley's figure (1909, fig. 4) the pelvis is restored by analogy with marsupials. The Eucosmodon 

 specimen shows this restoration to be incorrect. 



