1905.] CLAVICLE OP DIPLODOCUS. 293 



In Chelonia the penis sometimes exiiibits internally a partially 

 bifid structure. 



For the purpose of this paper the penis of birds is of quite 

 exceptional interest. In its origin it is not only traceable to the 

 Crocodilian type, but shows a very great amount of asymmetry, and 

 besides in tlie Ratitae a distal bifurcation of the corpus fibrosum 

 (text-fig. 48, p. 291). In Struthio the distal part of the penis is 

 changed into a gians-like organ, while in Rhea the corpus fibrosum 

 consists of an exceedingly hard and nearly cartilag-inous substance. 



A bifurcation like that observable in the problematical bone of 

 Diplodocus is also frequently to be met with at the distal end of 

 the mammalian os penis, which is often asymmetrical. The os penis 

 of mammals always shows quite remarka,ble variability. For 

 comparison with the bone of Diplodocus^ side and hind views of 

 the OS penis of the European Otter (Lutra lutra) are given (in 

 text-fig. 49, p. 292), and one can see at a glance the well-rounded, 

 smooth, condyle-like, distal ends, the proximal rugosities, and the 

 lateral impressions for the attachment of the corpus fibrosum. In 

 other mammals the corpus fibrosum is not attached laterally to the 

 ossified element, but ends in a deep pit situated at the proximal 

 end of the latter. 



We have therefore to consider the following propositions : — ■ 



(1) That among the Mammalia it is the corpus fibrosum with 

 which the os penis comes in close contact, forming the anterior 

 prolongation into the glans penis, that the os penis ossifies from 

 fibrous matter; that a corpus fibrosum is also present among 

 Reptilia, and that therefore an os penis in Dinosaurs can only 

 have originated from the corpus fibrosum. 



(2) That in Rhea the corpus fibrosum is quite as hard as cartilage, 

 and difters from this only by not possessing cartilage-cells. 



(3) That in Sauropsida a glans is frequently present. 



(4) That it is quite a common thing to find bird-like characters 

 in vaiious parts of the Dinosaui-ian skeleton. 



(5) That among the birds the Ratites show the most priniitive 

 and still the best-developed male genital organ. 



(6) That the shape and variation of the problematical bone in 

 D'qjlodocus are well in accord with its being an os joenis, while 

 they militate against its determination as clavicular. 



(7) Lastly, that this so-called clavicula when present is always 

 found only as an unpaired organ showing the same direction of 

 curvature. 



Hence I am of opinion that it is at present advisable to remove 

 the subject of this paper from the shoulder-girdle and determine 

 it as the ossified axis of the penis. 



Further evidence and, especially, fui'ther discoveries are naturally 

 necessary before so delicate a question can be regarded as definitely 

 settled ; but since Hatcher's single argument against the bone in 

 question being an os penis (namely, its asymmetry) breaks down 

 on reference to Struthio or even to Lutra, the balance of the 

 argument is at present in favour of this newer interpretation. 



Prog. Zool. Soc— 1905, Vol. II. No. XX. 20 



