VARIATIOX I\ RIBS OF PEXGUIX. 281 



nnl not connected with the vertebral colunm, it lias been lost or 

 overlooked. The sternal ribs are eight in number, completing 

 .the vertebral ribs from the third to the tenth. The eighth 

 -sternal rib, attached to the tenth vertebral segment, does not 

 reach the sternum, but is connected with the seventh sternal rib 

 and, according to Watson, is also easily overlooked on account of 

 its vestigial nature. 



The specimens were all dissected so as to display the 

 .sternum and the ribs on each side throughout their extent. In 

 most of them the proximal portion of the vertebral segments 

 was ossified, but the distal portion and sternal segments were in 

 .the cartilaginous condition. The series represented by Plates 

 VI — VIII, Figs. 1 to 9, indicate the chief variations met with. 

 In each specimen the eighth and ninth ribs are fully developed, 

 j^md reveal no important variations ; hence by representing the 

 tenth in its natural relationship with them, the degree of variation 

 is accurately displayed. 



In the specimen from which Fig. 1 is drawn no trace what- 

 ever of a tenth rib, vertebral or sternal, occurs. The ninth is 

 slightly more slender than usual, and its sternal portion is closely 

 united to that of the eighth for practically the whole of its length. 

 In Fig. 2 a narrow strip of cartilage, measuring about 5 mm., 

 Jies against the sternal portion of the ninth rib, and the condi- 

 tions represented in other specimens indicate tliat this is to be 

 regarded as a vestige of the tenth rib. Fig. 3 shows a longer 

 strip of cartilage in the same position, and in Fig. 4 it becomes 

 still longer, while in Fig. 5 it extends the full length of the sternal 

 'segment, closely united with it all the way. In the specimen 

 from which Fig. 5 was drawn a small isolated strip of cartilage 

 was found embedded in the muscles posterior to the ribs. Fig. 

 ■^) reveals the tenth sternal rib in its complete length, with a 

 vestige of the vertebral rib attached to it, but clearly separated 

 by an articulation. In Figs. 7 and 8 the sternal rib of the tenth 

 is also seen to be fully developed, but is very slender and for the 

 most part attached to the ninth, while the vertebral moiety is 

 uuich longer. Fig. 9 represents the furthest stage reached in any 

 of the specimens examined. On the right side the vertebral rib is 

 incomplete, while on the left the rib is fully complete in both its 

 sternal and vertebral portions, the latter connected with the 

 vertebral column by a tubercular and a capitular process. 

 Extending from the sternal segment is a process, slightly longer 

 on the left side than on the right, which is manifestly a vestige 

 of the sternal part of an eleventh rib, though no line of separation 

 from the tenth can be made out. 



Though the results have been arranged in this orderly 

 sequence, so as to show an almost continuous series between the 

 one extreme and the other, that is, between the entire absence 

 of the tenth rvl^ on the one hand and its complete development 

 on the other, it is by no means intended to imply that tliis is 

 the manner in which the variations have actually been produced 

 in Nature. It has often been pointed out that sequences of this 



