472 REPTILES. 



rib is completely invested by a tubular sheath of young connective tissue, 

 and in the intercostal spaces are distinct muscle plates. In the turtle the 

 cartilaginous ribs are simply embedded in young osteogenetic tissue, 

 which forms the whole of the body wall, extending superficially up to 

 the skin. As development proceeds in the crocodile, the tubular sheath 

 of connective tissue (periosteum) investing each cartilaginous rib, grows 

 in size, and forms bone (the rib) anterioryl, the cartilage being absorbed. 

 Thus we get the adult cylindrical rib, separated from its neighbours by 

 the intercostal muscles, developed from the muscle plates. In the 

 green turtle bone begins to form upon the rib cartilage, the latter sub- 

 sequently being absorbed, but as there is no investing periosteal sheath, 

 this formation of bone spreads out on all sides, right up to the skin 

 superficially, and as far as the neighbouring growths laterally, to form 

 the solid bone of the carapace. In the mud turtles, the growth of bone 

 which is extending laterally from each cartilaginous rib, does not meet 

 its neighbour, for already the intercostal tissue has partly become differ- 

 entiated into fibrous tissue, and a fibro-osseous carapace results. In the 

 green turtle, the rib cartilage, at both its distal and proximal ends, is 

 invested by true periosteum, which causes in these parts the formation 

 of cylindrical bone. 



What then is a costal plate ? 



It is more than a rib ; it is a rib, which, in its development, has spread 

 into and involved the surrounding intercostal tissue. 



Is it an intramembranous or intracartilaginous bone ? We now 

 know that all bones are developed through the agency of membranes, 

 and that the humerus, for example, an intracartilaginous bone, is 

 eventually formed entirely from its membranous periosteum. It differs 

 from the scapula (a membrane-bone) in this respect, that it was at 

 one time represented by cartilage, while the scapula was not. A mem- 

 brane-bone is therefore not a bone developed from a membrane, for 

 every bone in the body is now known to be so formed, it is a bone whose 

 place was never represented by cartilage. 



If we accept this view of an intramembranous and intracartilaginous 

 boiie, a view forced upon us by modern inquiry, then the costal plate is 

 an intracartilaginous bone, and comes out in its proper contrast from the 

 marginal and plastron plates which are not preforned in cartilage. Hoff- 

 mann, not appreciating the general facts relating to bone development, 

 argues that the costal plates are intramembranous because membranes 

 enter into their formation. The neural plates may be looked upon as 

 similar in their origin to the costal plates, bone encrusting the cartila- 

 ginous vertebrEE, and then extending into the tissue between neighbour- 

 ing spinous processes, and superficially up to the tissue which has 

 differentiated into the thin layer of connective tissue, which is subscutal 

 in its position. 



The ventral shield or plastron consists of dermal bones ; according 

 to some the three anterior pieces represent clavicles and interclavicle. 



The cervical vertebree have at most little rudiments of ribs, are 

 remarkably varied as regards their articular faces, and give the neck 

 many possibilities of motion. There are no lumbar vertebrffi. 



The bones of the skull are immovably united ; there are no ossified 

 alisphenoids, but downward prolongations of the large parietals take 



