684 REPTILIA. 



connected together by sutures resembling those of the human cranium. 

 The ribs are changed into broad flat bones, firmly united by suture to 

 each other and also to the lateral margins of the spinous processes of 

 the vertebrae, so that they all form, as it were, a single broad plate : the 

 heads of the ribs are very feebly developed, and the intervals between 

 them and the bodies of the vertebrae filled up with ligament. The 

 margin of the shield thus formed by the dorsal ribs is further enlarged 

 by a third set of flat bones, apparently representing the sternal ribs of 

 the Crocodile, fixed by suture around the whole circumference of the 

 carapax, which they assist in completing. 



(1910.) The plastron, or sternum, is made up of nine pieces, which 

 have been proved by M. Geoffroy St.-Hilaire to be the elements of this 

 portion of the skeleton in the most complete state of development in 

 which they are met with. Of these nine elements, eight are disposed 

 in pairs ; but the ninth, which is always placed between the four pieces 

 composing the two anterior pairs, is single, and occupies the mesial 

 line. In birds, we shall afterwards find this element of the sternum 

 performing a very important office. 



(1911.) The bones of the shoulder and of the hip, in the Tortoise 

 (fig. 336), are absolutely placed within the thorax, and articulated to 

 the sides of the vertebral column. The precise homology of the scapular 

 apparatus has not been as yet decidedly pointed out ; there are, how- 

 ever, three branches, probably representing the scapula, the clavicle, and 

 the coracoid bone : but in the construction of the pelvis, the ilium, the 

 ischium, and the pubis are identified with facility. 



(1912.) The muscular movements of Eeptiles are ordinarily slow and 

 languid, a circumstance which no doubt depends upon the impurity of 

 their blood, consequent on the imperfect manner in which the circulating 

 fluid is exposed to the influences of respiration. The muscles of these 

 animals are, however, peculiarly tenacious of life, and preserve their 

 irritability and power of contraction for an astonishing length of time 

 after they have even been separated from the body. The muscles of a 

 Turtle will continue to live for days after the creature has been decapi- 

 tated ; and the heart will still contract, when irritated, even many 

 hours after its removal. 



(1913.) But perhaps the most interesting phenomenon connected with 

 the muscular system of the Reptilia is the progressive development of 

 entirely different sets of muscles as the metamorphosis goes on by which 

 they are converted from their earliest fish- condition to their mature and 

 perfect state. This series of changes, which doubtless takes place in all 

 the higher Vertebrata, is well exemplified in the tadpole of the Frog or 

 Toad, and the different phases of development are in such creatures 

 easily investigated. At first the tadpole presents the muscular structure 

 of a fish, both in the muscles of the expanded and vertical tail and in 

 those of the branchial apparatus. As growth proceeds, the broad 



