366 CETACEA. 



time above the alveolus as a hard tube until it either breaks across or falls out. 

 The teeth of the lower jaw, as already indicated, are much larger than those of the 

 upper jaw, and are not so curved as the latter, the crowns of which are set in at a 

 markedly obtuse angle to the root. The crowns of the teeth are worn flat from 

 without inwards and (TOwnwards. The greatest diameter is obtained at the middle 

 of the tooth, and the fourth of the upper jaw is the largest ; the teeth behind it 

 decreasing in size to the last, which has a double curve and a conical, almost 

 unused crown. In the lower jaw the anterior tooth is quite as large as the teeth 

 behind it. 



Subjoined are the measurements of the teeth of the specimen of Orcella 

 fluminalis under consideration : — 



Upper jaw. Lower Jaw. 



Inches. Inches. 



1st tooth, right side, length 0"40 070 



» V breadth OIG 0-24 



4th „ „ length 0-62 0-68 



» » 5. breadth 0-20 0-20 



Last tooth, „ length 0'28 O'SS 



breadth 008 0-16 



Vertebral column. — There are in all 63 vertebrae, viz., C 7 : D 13 : L 16 and 

 Caudal 26. The atlas and axis are firmly soldered together (Plate XLIII, fig. 3), but 

 all the others, with the exception of the last cervical vertebra, are perfectly free. 

 The spinous process of these united vertebral elements is markedly bifurcate at its 

 extremity. 



These amalgamated bones of the two species (Plate XLIII, figs. 3 and 8) present 

 certain characters by which they can be at once distinguished. In O. fluminalis the 

 facets for the occipital are more widely divergent than in O. brevirostris, and 

 are separated at their inferior border by a much narrower interval. In the Irawadv 

 dolphin the facets are almost oval, but in O. brevirostris they are much narrower 

 below than above. The height and breadth of the neural arch in the latter are pro- 

 portionally less than in the former, but the greatest breadth of the two bones is nearly 

 the same in an individual of O. brevirostris with a skull measuring 11-72 inches, 

 and in the specimen of O. fluminalis with its skull 12-16 inches in length. In the 

 latter species the breadth of the spinous process below its bifurcation is much greater 

 than in O. brevirostris, and the antero -posterior extent of the lamina is much 

 greater in O. fluminalis. The remaining indications of the odontoid process are 

 also more strongly marked in the Irawady dolphin. 



The lateral laminae of the cervical vertebrae are most expanded in the 5th 

 segment, which is represented on Plate XLII, fig. 5, and which will suffice to give a 

 general idea of the characters of these bones. The total length of the cervical region 

 in the fresh skeleton is 4*50 inches. 



The first dorsal vertebra is figured on Plate XLII, ^g. 6, reduced to one-half 

 natural size, and along with it the 7th dorsal and the 1st lumbar or that vertebra 

 opposite to which occurs the 14th or free floating rib. In structure this vertebra 

 belongs essentially to the lumbar region. The transition from the comparatively 



