600 THE PECTORAL GIRDLE. 



prominent, and in the majority of cases the coracoid can hardly 

 be recognised. The coracoid process is however well developed 

 in the Selachioid Ganoids, and the Siluroid Teleostei. In 

 Teleostei the scapular region often ossifies in two parts, the 

 smaller of which is named by Parker praecoracoid, though it is 

 quite distinct from Gegenbaur's prsecoracoid. The membrane 

 bones, as they present themselves in their most primitive state 

 in Acipenser and the Siluroids, are dermal scutes embracing the 

 anterior edge of the cartilaginous girdle. In Acipenser there 

 are three scutes on each side. A dorsal scute known as the 

 supra-clavicle, connected above with the skull by the post- 

 temporal; a middle piece or clavicle, and a ventral or infra- 

 clavicle (inter-clavicle), which meets its fellow below. 



In most Fishes the primitive dermal scutes have become 

 subdermal membrane bones, and the infra-clavicle is usually not 

 distinct, but the two clavicles form the most important part of 

 the membranous elements of the girdle. Additional mem- 

 brane bones (post-clavicles) are often present behind the main 

 row. 



The development of these parts in Fishes has been but little 

 studied. 



In Scyllium, amongst the Elasmobranchii, I find -that each 

 half of the pectoral girdle develops as a vertical bar of cartilage 

 at the front border of the rudimentary fin, and externally to the 

 muscle-plates. 



Before the tissue forming the pectoral girdle has acquired 

 the character of true cartilage, the bars of the two sides meet 

 ventrally by a differentiation in situ of the mesoblastic cells, so 

 that, when the girdle is converted into cartilage, it forms an 

 undivided arc, girthing the ventral side of the body. There is 

 developed in continuity with the posterior border of this arc on 

 the level of the fin a horizontal bar of cartilage, which is 

 continued backwards along the insertion of the fin, and, as will 

 be shewn in the sequel, becomes the metapterygium of the adult 

 (figs. 344, bp and 348, nip}. With this bar the remaining skeletal 

 elements of the fin are also continuous. 



The foramina of the pectoral girdle are not in the first 

 instance formed by absorption, but by the non-development of 

 the cartilage in the region of pre-existing nerves and vessels. 



