550 Mr. Yarrell on the Osteologj/ 



part bent to form an arched plate of bone, the concavity of which 

 faces downwards and outwards ; the crista of great length from 

 before backwards. The inferior portion of the ilium is much 

 stronger, inclining outwards, from its junction with the sacrum 

 to the acetabulum. 



The transverse and spinous processes of the sacrum are repre- 

 sented by three slender plates of bone, which, approximating as 

 they pass backwards, are united to form a septum, extending 

 down the median line of the sacrum to the tail. A channel is 

 formed on each side of this septum by a thin flat plate of bone, 

 which, arising from the posterior and superior part of the is- 

 chium on each side, is bent over the back part of the sacrum, 

 and fixed to an arched and prominent plate of bone, which is 

 extended from this septum outwards, to form a junction with it. 

 The channels thus produced are bounded below by the sacrum, 

 on the inner sides by the septum, on the outer sides by the 

 ascending plates of bone just described, and above by the junc- 

 tion of both. From this union a short osseous stem issues hori- 

 zontally on each side, and expands into a flattened circular 

 plate of bone, to the rough surface of which, as well as to the 

 tuberosity of the ischium below, portions of the truncated exte- 

 rior of the animal are firmly attached. 



The under surface of the sacrum is broad and flattened, and 

 marked by an indistinct central ridge, as shewn in figure 7. 

 The pelvis is open in front, the ossa pubis on each side do not 

 incline inwards, but descend at right angles from the horizontal 

 surface of the sacrum. In the circumstance of the pelvis being 

 open, there is a second resemblance to the bony structure in 

 birds. 



The caudal vertebrae are fourteen in number ; the transverse 

 processes of the last four are elongated, to support the thin di- 

 lated lateral edges of the paddle or spatular extremity of the 

 tail. Large muscles are imbedded in the two cavities formed on 

 the upper surface of the sacrum by its septum and the two 

 lateral elevated portions of the ischium before described ; and 

 there are antagonist muscles of equal size on the under surface. 

 The tendons of these muscles were inserted on the upper and 



