394 REPTILES xv. 3 



and ribs, but not actually formed from the latter. The plastron is 

 developed from the expanded dermal bones of the pectoral girdle, 

 together with dermal ossifications comparable to the abdominal ribs 

 found in crocodiles and other reptiles. The whole is covered in most 

 chelonians by rows of special smooth epidermal plates forming the 

 'tortoise shell' (Fig. 225). A new and larger layer is added to each of 

 these plates each year, the old one remaining above it, thus making a 

 number of 'growth rings' from which some indication of the age of the 



Neur. S. 



Neur. PL. 



Mar (j. S. 



Pect. S. 



Fig. 225. Diagram of the arrangement of the shell of the tortoise 



(Testudo). The horny shields are shown only on the left. 



cap. capitulum of rib; cost.pl. costal plate; cost.s. costal shield; 7tiarg.pl. marginal 



plate; marg.s. marginal shield; ncur.pl. neural plate; tieur.s. neural shield; pect.s. 



pectoral shield; plast. plastron; sp.c. spinal cord. (After Gadow). 



tortoise can be calculated, though the outer members often become 

 rubbed off. 



In order to support this box the limb girdles have become much 

 modified and lie inside the encircling ribs. The pectoral girdle has 

 three prongs, a scapula that meets the carapace dorsally and carries a 

 long 'acromial process' and a backwardly directed coracoid, the two 

 last being attached bv ligaments to the plastron. The ilia are attached 

 to two sacral vertebrae and the ischia and pubis are broad. The limbs 

 are stout, but otherwise typically reptilian, with five digits in each. 

 In the marine turtles they are transformed into paddles. 



The interpretation of the skull is still somewhat doubtful, but it 

 seems not unlikely that the turtle, Cheloue, shows the simplest case, 

 namely, the original anapsid condition (Fig. 226). Here the roofing is 

 complete, the dermal bones being widely separated from the brain- 

 case, forming a tunnel for the jaw muscles and those producing 

 retraction of the neck. The tympanum is stretched across a sort of otic 

 notch, bounded by the squamosal, quadrato-jugal and quadrate, the 



