OCR Text |
Show 1905.] CLAVICLE OF DIPLODOCUS. 293 In Chelonia the penis sometimes exhibits internally a partiallv 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 the Ratitaj a distal bifurcation of the corpus fibrosum (text-fig. 48, p. 291). In Struthio the distal part of the penis is changed into a glans-like organ, while in Rhea the corpus fibrosum consists of an exceedingly hard and nearly cartilaginous 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 remarkable 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 differs 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 various parts of the Dinosaurian skeleton. (5) That among the birds the Ratites show the most primitive and still the best-developed male genital organ. (6) That the shape and variation of the problematical bone in Diplodocus are well in accord with its being an os penis, 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, further 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. P roc . Z o ol. S oc.-1905, V o l . II. No. XX. 20 |