5(32 THE NATURAL HISTORY REVIEW. 



jecting azygous portion of the primordial episternals — a process 

 which is seldom developed in the Mammalian class. That a division 

 may really take place in an originally simple piece leading to the 

 formation of an episternal apparatus composed of a central and two 

 lateral portions, is clearly shown in Coelogenys. It is consequently 

 highly probable that all symmetrical episternals correspond not to 

 the whole, but only to the lateral portions of the primordial piece, 

 so that we cannot regard them as resulting from the division of a 

 common median piece, but rather as a degradation in point of deve- 

 lopment of the latter leading to its total disappearance, whilst the 

 lateral portions still remain persistent. But remains of the median 

 piece united with the sternum have been shown to occur in 

 the Mole. (Fig. 8.) Such remains lying in front of the manubrium 

 sterni, and consequently between the above-mentioned symmetrical 

 pieces, may in some cases assume a cartilaginous or an osseous form, 

 and naturally lead to the formation of the ossa supra-sterndlla.^ 



From these considerations it appears to me that the latter are 

 abnormally occurring rudiments of a median episternal piece, 

 through which, as happens so frequently in other animals, we are 

 reminded of lower grades of organisation. Having thus examined 

 the relations of the episternals in what I must admit to be a very 

 limited number of mammalian genera, it will be advantageous to re- 

 view generally their points of agreement and diiference, and thus to 

 gain some insight into the relations of these several structures to one 

 another in Mammals, and to compare them Avith the analogous 

 structures that are found in the other vertebrated classes. It will 

 be advisable to commence with those of the Eeptiles. 



In the Lizards the episternals appear as a T-shaped or cruci- 

 form bone, of which the median portion overlies the proper sternum 

 to a considerable extent. In the Crocodiles the transverse branches are 

 wanting, and hence the episternal corresponds, independently of its 

 position in regard to the sternum, which is the same as in Lizards, 

 rather to the most anterior segment of the sternum of Seals, or 



* Though these are found to be arranged symmetrically, it hy no means follows 

 that they must eorrespond with parts that are originally symmetrical : for the 

 history of the development of the sternum renders it evident that symmetrical 

 osseous nuclei may occur in an asymmetrical portion of cartilage. Moreover, if we 

 admit that the first appearance of the asAinmetrical episternal piece and the mode 

 of its ossification when it becomes bony are wholly unknown, it is clear that the sym- 

 metrical arrangement of the supra-sternal bones is by no means opposed to the 

 belief that they may be homologous with a portion of the skeleton that is elsewhere 

 asymmetrical. 



