01^ THE episte-j[i:n-al apparatus of mammals. 5G5 



times appearing as a longer piece (as in the Carnivora provided with 

 clavicles, and as in Myrmeeophaga didactgla), and sometimes as a 

 shorter piece as in Sciurus ; or it proceeds from only a limited portion 

 of the terminal surface, and thus permits the remaining greater por- 

 tion of the clavicle to be directly articulated with the sternum, as 

 in the Yolitantia. 



Thus from the varieties in the size, form, and intimate structure 

 of the episternals in certain Mammalian classes, it follows that 

 besides the proof of the general existence of these parts of the skele- 

 ton, there is evidence that there may be found within the limits of 

 each class a series of modifications : yet however wide these devia- 

 tions from the common type may be, there does not appear to be 

 any difficulty in recognizing in the structures in question a some- 

 thing which is common to all the Unguiculata. 



In the examination of the anterior sternal segment of some of 

 the above-mentioned animals, I have observed another peculiar rela- 

 tion, which I shall here briefly mention. In Mice, and especially in 

 Mus musculus (see Eig. 3) an oval-shaped cartilage is visible at the 

 side of the expanded portion of the most anterior segment of the 

 sternum. On examining it from the front, only one half of it is 

 visible. This piece possesses a sharply defined contour in its whole 

 circumference, up to the very point where it is applied to the sternum, 

 and the point of application is always exactly intermediate on each side 

 between the episternal cartilage and the attachment of the first rib. 

 On superficial examination we might consider it to be a peculiar and 

 independent portion of the skeleton, and when I first discovered it 

 I thought that I had obtained in this little cartilaginous piece the 

 rudiment of the second or posterior clavicle, which is developed as 

 far as the sternum only in Monotremata amongst Mammalia, appear- 

 ing elsewhere as a scapular appendage (processus coracoides). The 

 investigation of the mode of connection of this cartilage with the 

 sternum led me to doubt this view, for on making transverse sec- 

 tions I found there was not a mere application of the two surfaces 

 at this point, but a connection of a much more intimate nature. 

 Additional circumstances of some weight in determining the morpho- 

 logical significance of this sharply defined cartilaginous mass, which 

 projects from the transverse branch of the manubrium sterni, but 

 does not appear to be merely a prolongation of it, are, that it is of 

 very constant occurrence, and that its ossification does not proceed 

 continuously from the sternum, but commences from a perfectly inde- 



