EPISTERNUM 



63 



Fig. 47. — Pectoral Akch op PAtasoHAT- 

 TERiA (from the ventral side) After 

 Credner. 



8, scapula ; 



O, coracoid ; d, clavicle ; 

 episternun. 



SlJS, 



lu the StegocejAala, the episternum (" interclavicle ") consists 

 of a large bony plate, situated ventrally to the sternum, some of 

 the various forms of which, 

 as "well as its relation to 

 the pectoral arch and more 

 particularly to the clavi- 

 cles, will be seen by refer- 

 ence to Fig. 46. 



The episternum of 

 Palceohatteria and of re- 

 cent Lizards and Crocodiles 

 is essentially similar to 

 that of the Stegocephala 

 (Figs. 44, 46, and 47). In 

 Lacerta ar^d Crocodilus it 

 arises, from before back- 

 wards, as a paired structure, , 

 which is not preformed in 

 cartilage. An epistermim 

 is wanting in Chelonia and 

 Ophidia, as well as in 

 Chamseleo and Anguis. 



In Birds no independent elements corresponding to this 

 structure can be recognised ; the ligament extending between 

 the clavicles and the sternal keel, the periosteal covering of the 



keel which is continued -backward 

 from this ligament, and the median 

 portion of the fused clavicles when 

 separately ossified ("interclavicle") 

 may possibly have something to do 

 with an episternum without being 

 exactly homologous with it. 



The origin and meaning of the 

 mammalian episternum, which is pre- 

 formed in cartilage, is not known ; it 

 has probably no direct connection 

 with the similarly-named structure in 

 Keptiles, but apparently agrees with 

 the latter at any rate as regards 

 position and relations in the embryo 

 Mole (Fig. 47a). 



In Monoiremcs (Fig. 48) and certain 

 Marsupials a median and two lateral 

 portions can be distinguished, the 

 latter being in connection with the 

 clavicles. In these Marsupials the 

 median portion unites with the ster- 

 num, and as in Monotremes, becomes 



Fig. Ha. — Episternum of an 

 Embryo Mole. (After 

 A. Giitte). 



St, sternum ; es^, central por- 

 tion and es", lateral por- 

 tion of the episternum ; 

 d, clavicle; r.c, ribs. (The 

 figure was constructed 

 from two consecutive hori- 

 zontal sections.) 



