444 



COMPARATIVE ANATOMY. 



A special piece, the episternum, is very commonly present in 

 connection with the sternum ; this has one of two forms according 

 to the way in which it is developed and connected. 



In one the episternum is merely represented by bony structures, 

 which are placed on the ventral face of the sternum. Thus, in the 

 Reptilia, it has the appearance of a cross, or of a T-shaped bone 

 (Fig. 238, t), the two arms of which support the clavicles, while the 

 median piece is attached to, or even fused with, the sternum (Asca- 

 labota). In the Crocodilini the transverse piece of the episternum 

 and the clavicles are both wanting, as is the whole episternum in 

 the ChamaDleonida) ; nor is it present in Birds. 



The second type of these structures is made up of skeletal parts, 

 which are preformed in cartilage, and which lie in front of the 

 sternum. There is a structure of this kind in the anourous 

 Amphibia (Fig. 232, e), where it is represented by a bone, which is 



separated from the sternum by the 

 coracoid, and is therefore set in 

 front of the shoulder-girdle. 



In the Mammalia the episternum 

 always forms a uniting piece be- 

 tween the sternum and clavicle. 

 In the Monotremata it has the 

 form of a bone attached to the 

 sternum, and drawn out into two 

 processes. In the Marsupialia (Di- 

 delphys) the lateral arms (Fig. 238) 

 remain movable, but the median 

 portion is fused with the sternum. 

 This leads to that reduction of the 

 episternum which is seen in other 

 Mammals ; in which the lateral 

 pieces alone are present in cartilage, 

 or form bony parts, which are attached to the sternal end of the 

 clavicle (Rodentia, Insectivora, Edentata). In the Primates these 

 episternal structures form the intermediate cartilages of the articu- 

 lation of the sternum with the clavicle. 



Gegenbadr, C, Ucber die episternalen Skelettheile mid ihr Vorkomxnen bci den 

 Sangethieren und beim Menschen. Jen. Zeitschr. I. — Parker, W. K., Structure 

 and development of the Shoulder-girdle and Sternum. Ear Soc. 1868. 



Fig. 238. Episternum, with the parts 

 connected to it, of a young Opossum. 

 st Anterior end of the sternum (ossi- 

 fied), ep Episternum (cartilaginous). 

 cl Clavicle, c The first two ribs. 



Cephalic Skeleton. 



§339. 



The indifferent stage in which the head is found in the Acrania 

 makes it impossible to distinguish any distinct cephalic skeleton. 

 It is, however, true that the head of the Craniota is not an absolutely 



