THE OSTEOLOGY AND MYOLOGY 



Fin 



16. — Upper and side views of Sa- 

 crum anil first Coccygeal. 



dried preparation of the pel- 



of wliiL-h it may be be.st to consider it. The chief modification of lumbar characters that 

 the two sacrals undergo is exhibited in their transverse processes, which thicken in every 

 direction, and become confluent, leaving only a moder- 

 ately large circular foramen at the base of their line 

 of union. While the expanded and elongated trans- 

 verse process of the last lumbar points obliquely down- 

 ward-forward, and upward-backward, the reverse is the 

 case with the similarly expanded and oblique, though 

 thickened, sacral diapophyses. This change is necessary 

 for coaptation of the osseous surface with the obliquely 

 placed iliac shaft. The synchondrosial surflice borne 

 upon the confluent diapophyses is irregular in shape, 

 somewhat approacliing a narrow semilune. Union with 

 the ilia is very imperfect • so slight is the force required 

 to detach the sacrum, that some care is requisite in making 



vis. This loose-jointedness is of a part with that at the symphysis menti et pubis, among 

 many of the cranial bones, the several sternebers, etc. ; and is, in fact, one of the char- 

 acteristics of the animal's skeleton. The spinous processes of the sacrals remain distinct 



from each other ; they have the same shape as those of the last two 

 lumbars, and incline slightly forward. Zygapophyses are perfectly 

 developed on both of the bones, but they only subserve movable 

 articulation with the last lumbar, in consequence of the anchylosis 

 of the two sacrals, and of the last of these with the first caudal. 

 The motion of the last lumbar upon the sacrum is c^uite free. 



Caudal Vertehroe. — (Fig. 17.) Admitting the above mentioned 

 questionable vertebra to this series, the tail is composed of twenty- 

 three bones.-^ The adaptation of these bones to the prehensility of 

 the member results in some interesting modifications. The first, 

 as we have seen, is (always?) anchylosed with the sacrum ; the re- 

 mainder are freely movable, yet most of them — all beyond the 

 fifth — have a different articulation, in which zygapophyses take no 

 part. The series of articular surfaces is so arranged that the tail, 

 when not in use, habitually assumes the shape of a note of interroga- 

 tion laid horizontally, thus, c^-, as maybe shown by boiling the bones 

 from which the skin and muscles have been dissected, and allowing 

 them to dry, suspended in the air by their proximal extremity. 

 Part of the curling under of the tail in life is, however, due to trac- 

 tion of the flexor tendons. Near the root of the tail some downward 

 convexity is possible ; elsewhere, the member cannot be extended 

 Pig 17 beyond a straight line without forcible bending. Lateral bending is 



Coccygeal vertebrfe: A, everywhere more or less free ; and particularly so towards the root 

 i2tii; D^'istii" '' ' of the tail; so that when the animal is squatting, the tail can be 



1 Owen (Comp. Anat. and Phys., ii, p. 332), says caudal vertebrie varies within wide limits among the differ- 



" twenty-two." The same author assigns to another species ent species ; and in the same species a ditTerence of one or 



of the genus — D. cancr'wora — thirty-one. In this, therefore, two bones may possibly occur, .as an individual pecuharity. 



as in other genera of long-tailed marsupials, the number of ' First unanchylosed one. 



