REPTILES. 139 



question, especially of the anterior pair, and thus exhibit an inverse relation as regards position to 

 that which is met with in the higher Vertebrata. Moreover, in the Marine species, and in Trionyx, 

 they are not attached by one extremity to these pieces, but advance to the middle line of the body to 

 a greater or less extent over them, varying with the particular species and the age ; consequently, in 

 the Chelonians in general the relation of the great pectoral muscles to the Plastron is neither so 

 intimate, or necessary, as that which they possess to the Sternum in the higher Vertebrata. Hence, 

 as from the grounds I have adduced the original rod-shaped lateral parts of the Plastron of the 

 Chelonians cannot be regarded as equivalent to those portions of the skeleton which constitute the 

 Sternum, neither can the azygous piece of the Chelonian Plastron possess the significance of the 

 Sternum. For the latter forms itself within various muscles, and always maintains its place amongst 

 them, like the ribs and other parts of the animal skeleton, whilst the Plastron of Chelonians generally 

 is formed independently of all muscles. 



Secondly, the Plastron develops, as has been already fully described, in the subcutaneous connective 

 tissue, and is originally completely concealed within it. For, according to observation made on Testudo, 

 and Marine Chelonians, it lies originally, and for a considerable period, so enclosed in the substance 

 of the thick and dense layers which are composed of the subcutaneous connective tissue, that it is 

 entirely invested by them, not only on its upper, but also on its lower surface. 



The Plastron thus originates in the same matrix as that in which the accessory (supplemental) 

 plates of the carapace of the Chelonians is formed, and as that in which the bony plates of the skin of 

 many other Vertebrata is developed, as for instance, in the Crocodiles, Sturgeons, and Syngnathidse 

 (as I can state from my own researches), whilst the vertebrae and the ribs more distant from the 

 surface of the body develop from another kind of matrix. It is obvious from these genetic relations 

 what significance is to be attributed to the Plastron. Like the supplemental plates of the Carapace, it 

 is to be regarded as essentially a portion of the dermal skeleton. 



Note. Hence, I hold that the comparison made by Geoffrey St. Hilaire the elder, in his ' Philosophical Anatomy ' 

 (vol. i, p. 106), between the several pieces of the Plastron of Chelonians and the osseous pieces composing the Sternum of Birds 

 is opposed to nature, and wholly fails. 



All the original figures in Plate XII (1 9) are from newly hatched young of Chelone my das ; 

 fig. 8 shows a transversely vertical section through the anterior lip of the plastron ; fig. 9 a little 

 further back, and fig. 7 posterior to it; the last section has cut through the inter-clavicle, the 

 other two through the clavicles only. It is clearly shown that the two laminae of bony matter are 

 mere calcifications of layers of the skin (c. v.) ; where these deposits have begun to form a suture 

 by the interlocking of the right and left clavicle (see fig. 9), a third lamina has commenced, and these 

 delicate bony layers have coalesced at this point. The space between the bony laminae is filled 

 with a softer fibrous stroma than that which forms the corium, and the connective tissue inside 

 the new bone is of a looser texture. These ossified tracts have already taken on very much of 

 their permanent form (see fig. 1, where the relation of the upper surface of the plastron to the 

 Shoulder-girdle is displayed). The two foremost (cl.)form an elegant horizontal arch; the single 

 piece (i. cl.) is nail-like, and its broad end is locked in between and behind the front pair; then 

 come two larger radiating plates (p. 1. 1.), overlapped in front by the foremost bones. On each 

 side of the umbilicus (u.) there is another pair of many-spurred bones (p. r. a.) ; and wedged between 

 their hindermost forks there is, on each side, a flat bony plate, pointed in front, and rounded behind 

 (p. t. a.). The special names for the three foremost are " clavicles" (cl.) and " interclavicle" (i.cl.) ; 

 their general names are " prae-thoracic " and "inter-thoracic;" the next pair (p.t.) are the "post- 



