OEOELLA. 367 



short transverse processes of the dorsal vertebrae to the long processes of the lumbar 

 region is effected in the 13th dorsal in which the process suddenly lengthens, having 

 its external half directed obliquely backwards. There is also a decided increase in 

 the length of the spinous process of the 13th dorsal vertebra, and it, along with the 

 transverse process, attains its maximum development on the 3rd lumbar, and from 

 that point both gradually decrease in size until the spinous process is lost on the 

 15th caudal and the transverse process on the 11th caudal. The neural canal 

 is perfect to the last process. From the 16th caudal the vertebrae rapidly diminish 

 in size, chiefly in their vertical depth, their transverse diameter more slowly 

 decreasing, until in the last segment only a round nodule remains 0'15 inch in breadth. 

 This little ossicle lies close below the skin in the notch between the caudal flippers. 

 The 5th and 6th caudal vertebrae are perforated as usual by a canal passing through 

 the base of the transverse processes, but in the succeeding vertebrae the canal opens 

 on the under surface of each segment in the broad vascular furrow protected by the 

 chevron bones, but is also present in the 4th, 5th, and 6th caudals. 



The chevron bones commence between the 12th and 13th lumbar vertebrae, — that 

 is, anterior to the position of the pelvic bones, — and have the general Cetacean 

 character. Their spines are well developed as far as the 14th caudal, but beyond 

 that they become reduced to a ridge and the bones themselves to nodules that can 

 be traced to near the end of the column. 



Bibs. — There are in all thirteen true ribs (Plate XLII, i to xiii) on either side? 

 but a free floating rib occurs on both sides considerably removed from the vertebral 

 column and lying free in the lateral abdominal muscles. Five are attached to the 

 sternum by thoroughly ossified sternal ribs, the last being attached to the posterior 

 border of the sternum on either side of the notch. The 9th rib and those following 

 it are borne only on the transverse processes, the transition occui-ring rather suddenly 

 at the 8th vertebra. 



Sternum. — This bone (Plate XLIII, fig. 10) differs considerably from that of 

 0. brevirostris (fig. 5), more especially in the dilated character of that portion to 

 which the first sternal rib is attached and in the full rounded sweep of that border 

 which in 0. brevirostris is concave. It is also distinguished from the latter by the 

 presence of a deep notch which separates the anterior wings. In the sternum of 

 O. brevirostris, however, there is a rounded hole occm'ring near the anterior margin 

 of the sternum in the mesial line which may mark what may have been the base of 

 a notch in the young skeleton afterwards closed in by the ossification of the 

 neighbouring margins. In O. Jtuminalis the posterior end of the sternum is 

 sometimes divided in two by a deep wide notch, and in O. brevirostris there 

 is occasionally an indication of such a division. The sternum of the latter is also 

 relatively broader than that of O. fliiminalis. 



Scapula. — The scapula of O. fluminalis differs from that of the marine form of 

 Orcella (Plate XLIII, figs. 2 and 6) in the greater length and expansion of the 

 post-scapular area, and in the more oval form (figs. 2 and 7) of the articular 

 surface for the head of the humerus. 



