ON THE EPISTERNAL APPAEATUS OF MAMMALS. 549 



of the cartilage are attaclied the broad flat ends of two outrunning 

 cartilages (ep), which, after gradually assuming a cylindrical form, 

 are applied to the clavicles in a manner that is precisely similar to 

 that which has just been described as occurring between the median 

 piece and the sternum. The histological diiferentiation is also here 

 present: and the lateral cartilages, which are more intimately con- 

 nected with the clavicles than with the median episternal piece, are 

 at first composed of hyaline cartilage, and gradually become con- 

 verted into iibro-cartilaginous structures. 



On comparing this arrangement with that found in opossums and 

 in monotremes, it is obvious that what in them is the azygous portion 

 of bone or cartilage with two rami diverging from it anteriorly, is 

 here split into three pieces : of which the median fragment is applied 

 to the sternum, and has therefore identical relations in all ; whilst 

 the transverse branches having become detached (as indeed occurs 

 to some extent in the Marsupials) are only brought into connection 

 with the median fragment by an attenuated portion, and are still 

 more intimately connected with the clavicle. The single episternum 

 of the Monotremes and of the Saurians is consequently here broken up 

 into three pieces, connected with one another by loose ligamentous 

 bands only, but much more firmly bound to other parts with which 

 originally they were simply in apposition. In Cavia, to which an " in- 

 complete clavicle " has been ascribed, a very simihir modification of the 

 episternal apparatus is present. In point of fact the skeletons I have 

 had the opportunity of examining, were altogether destitute of cla- 

 vicles ; but in front of the broad manubrium sterni, to the sides of 

 wdiich the first pair of ribs was attached — a peculiarly formed small 

 and flat bony structure appeared, which, in the recent skeleton, 

 would indubitably correspond to the ligamentous or cartilaginous 

 lateral portions of the episternal apparatus of Coelogenys. At any 

 rate the median piece is completely homologous in both animals. 



Hysirix {H. crisfcda) also certainly possesses an episternal ap- 

 paratus. In one skeleton of this animal I found a cartilaginous 

 mass at the anterior extremity of the sternum, behind which the long 

 cartilaginous episternum was connected with the corresponding rudi- 

 mentary clavicle. In the interior of the episternal piece I observed 

 a point of ossification. In the Tuoii&e {3fics musctihts, see Kg. 3). 

 the rounded sternal extremities of the clavicles are continuous on each 

 side with a portion of cartilage, of about yVth of an inch in lengih. 

 These fragments of cartilage converging towards one another become 



