INIA GEOFFEENSIS AND PONTOPOEIA BLAINVILLII. 101 



an extraordinary deviation from all other Cetaceans, among which the number, though 

 certainly very variable, is usually considerable, ranging from eight in Platanista and 

 Physeter to twenty-four in some of the Belphini and LagenorhyncM. On the other 

 hand, in the Sirenia, tlie lumbar region of tlie vertebral column is, as in Inia, extremely 

 restricted. 



The three lumbar vertebrte are very remarkable for the great antero-posterior 

 breadth of tlieir processes, both spinous and transverse. The bodies are large, being 

 respectively 1'3", l-i", and 1"5" in length ; their extremities are subcircnlar, and, as 

 usual in the Cetacea, the middle of the side below the origin of the transverse process 

 is much contracted, so that the median line of the under surface forms a sharp ridge, 

 from which a strongly marked arterial groove runs outwards and backwards to the 

 hinder edge of the root of the transverse process. The spinous processes resemble 

 those of the posterior dorsal region ; the first two are slightly curved forwards, the last 

 is nearly vertical and somewhat smaller. The oblique processes (metapophyses) are 

 short, flat, rounded projections from tlie upper part of the laminae of the arch, very 

 closely approximated to each other. The transverse processes rise from the whole 

 length of the side of the body ; they are of nearly equal lengtli, but increase in breadth, 

 especially by the development of a considerable angular process on the middle of their 

 anterior border, most conspicuous in the third vertebra; beyond this process the 

 anterior border is sharply cut off, so that the extremity appears to point backwards. 

 The hinder border is nearly straight, with a notch close to its origin from the body, 

 continuous with the groove before spoken of on the inferior surface of the bone. 



The vertebra here reckoned as the first caudal closely resembles the last lumbar. 

 Its body is of the same length, but its transverse process is even broader. The suc- 

 ceeding tail-vertebrae keep up the same general character, having large heavy bodies 

 and broad processes. The projecting surfaces on the hinder edges for the attachment 

 of the chevi-on bones are very strongly marked as far as the ninth, after which they 

 become obscure ; they are not seen on the anterior edge until the fifth. It is difficult 

 to determine exactly how many chevron bones there were, but probably not more 

 than eleven. The spinous processes, broad and rounded at their summits, become 

 gradually lower, until in the tenth the greatly reduced vertebral canal is scarcely closed 

 in by the lamina of the neural arch, and there is no longer a true spine. In the 

 eleventh, the canal is altogether open above. The metapophyses continue in much the 

 same relative development and situation as far backward as the spinous processes 

 extend. The transverse processes gradually diminish in length, and lose their charac- 

 teristic form. Already in the second that cutting away of the anterior edge noticed in 

 the lumbar region is lost ; and in the third and succeeding vertebrae the anterior edge is 

 straight, and the hinder one sloping, so that they appear to point forwards. In the 

 eighth they form but a slight prominence on the anterior part of the body, and in the 

 ninth they have altogether disappeared. The vertical perforations for the lateral 



