TEGUMENTS OF KEPTILES. 561 



pigmental cells are blended with the tissue of the shields and 

 scales, and may ornament the former with well-marked patterns, 

 e.g. in Testudo areolata, Ernys ornata, Emijs picta, &c. 



Cutaneous glands or follicles open between the warts of the 

 skin in Chdydra, and probably occasion their musky odour : but 

 this, in other Clielonia, appears to 1)6 due to larger, more com- 

 pact, and more localised glands. Beneath the epiderm of the 

 skin of the under part of the body, in the Soft- Turtles, is an 

 extensive network of vessels, spreading into dendritic ramifica- 

 tions, too numerous and large for the mere nutritive purposes of 

 tlic skin or sujiply of epithelial cells, and therefore probably ' 

 subservient to respiratiou. 



Viewing the integuments in their relations to the external 

 influences from which they defend tlie body, and by which they 

 are themselves affected, we may remark that most of the housc- 

 Ijearing Reptiles which have the surface of their abode habitually 

 in contact with air or water have the epiderm hard and thick, 

 whilst those living in ooze or mud have it soft and thin. In the 

 sea the horny scutes may be partially loosened, and grow over 

 one another : in the air they condense upon the surface with the 

 margins in contact : in the mud the skin is lubricous : the only 

 known scaleless species of marine habits (Sjiharyis coriacc(i) has 

 the tegument tough and leathery. In the most vagrant and widely 

 diffused Turtle (Chelone imliricaUi) the separation of the scales 

 takes place at a part of their circumference, which makes the 

 direction of iml^rication the most favourable ti.i their aquatic 

 movements. Growing and projecting from before l^ackward, the 

 one in front overlapping the next behind, the polished shields 

 offer no resistance to the forward movement impressed upon the 

 body by the oar-shaped limbs ; whilst the scutal interspaces, 

 widening as the trunk tends to recede during the preparation for 

 the next stroke, oppose the backward slipping, and take hold, so 

 to speak, upon the wave, retaining the advantage of one stroke 

 until the next is played. 



' As conjectured by Agassiz, ccc. vol. i. pt. ii. p. 284. 



VOL. I. 



