256 DISCOVERY REPORTS 



Third year (lo). Body length (3) 303-7 cm.; skull length 402-9 mm. ; zygomatic 

 width 213-5 mm.; alveolar length 103-8 mm. The palatal sutures are beginning to fuse, 

 most frequently starting with the maxillo-palatine followed by the median palatine and 

 the palatine part of the maxillo-premaxillar. 



Fourth year and over (9). Body length (5) 328-8 cm.; skull length (8) 41 1-5 mm.; 

 zygomatic width (10) 232-6 mm.; alveolar length 105-2 mm. The palatine sutures are 

 fused and obliterated, the premaxillae are fused to the palatal and alveolar parts of the 

 maxillae, and this fusion is continued along the vertical suture between these bones as 

 far as half-way up, but the upper parts of these sutures are not fused in any skull so far 

 examined. 



OS PENIS 



(Plate XI) 



The penis bones were not examined until it had been decided that there were four age 

 groups in the skulls, but when they were, they clearly provided an additional means of 

 distinguishing age groups. The smallest bone is that belonging to 325 ni, which is the 

 smallest male skull, 313 mm. (first year). This os penis is 96 mm. long and about 2-5 g. 

 in weight; it is curved in a dorsal direction with the maximum flexion about 34 mm. 

 from the distal end, but beyond this point the axis of the bone returns to approximately 

 its original line. The ventral surface has a narrow urethral groove for most of its length 

 and there is a perceptible dorsal keel. The bone belonging to a second year animal, 

 no. 657, is 132-5 mm. in length and 7-2 g. in weight; it is straighter than the bone of 

 325 m, but the distal part bends slightly downwards from about 38 mm. from the tip; 

 the dorsal keel is less sharp, and about 40 mm. from the proximal end there are two 

 elevations which mark the insertion of the ischio-cavernosus muscles. 



The OS penis of the third year, no. 664, exhibits a very marked increase in size since 

 it is 180 mm. long and weighs 27-5 g. and is altogether a more massive structure. For 

 most of its length the urethral groove is represented only by a flattening, but the dorsal 

 keel is well marked. The sudden increase in size and weight compared with the bone of 

 the preceding year — an increase of almost 36 per cent in length and of 380 per cent in 

 weight — supplies adequate reason for belief that the third year is that which marks the 

 onset of sexual activity, and this is in accordance with the structure exhibited by the 

 pertinent testicular sections, that of no. 657 being juvenile in character but no. 664 

 showing definite signs of the adult condition. 



The remaining four bones are the massive structures of the adult, although there are 

 indications that with sufficiently long series at least one additional growth stage could 

 be distinguished ; the bones of nos. 662 and 655 are deeper and more compressed than 

 those of nos. 665 and 666, with the ventral surface edged rather than flattened as it is 

 in the last two. 



To summarize, the os penis shows a continuous increase in size up to that of the 

 adult, and there is a sudden enlargement in the third year when the animal first becomes 

 sexually active ; there is also a progressive alteration in shape from one which is curved 



