554 ANATOMY OF VERTEBRATES. 



regular in size and shape along the back and sides of the body ; 

 but are large and transversely extended across the under part, 

 forming what are termed the ' ventral scutes,' ' scuta ventralia,' 

 the use of which in locomotion is explained at p. 259. All or 

 most of the scutes below the tail (' scuta sub-caudalia ') are single 

 in Crotalus, Bungarus, Boa ; in Python, the Colubridcc, and most 

 other serpents they are ' paired,' or divided along the middle line. 

 In most sea-snakes the abdomen is compressed and keeled below ; 

 in Pelamys the keel is bordered by two rows of scales ; in Hydro- 

 ]>hys it is formed by large bituberculate scales ; Platurus has the 

 venter scutate, with the caudal scales in pairs. Larger scales 

 occur in the head of most serpents, and serve as zoological cha- 

 racters, being defined as ' scuta marginalia labii superioris seu in- 

 ferioris,' 'scutum labiale medium,' 'scuta mentalia,' ' scuta ocularia,' 

 ' scuta frenalia,' ' scuta nasalia,' &c. The scales of serpents may 

 be smooth or carinate, they are rarely tuberculate (dorsal scales of 

 Xenodermus) ; and in their disposition they may be either ' con- 

 tiguous,' or ' imbricate.' 



The epiderm is condensed into claws or hooks (' calcaria ') upon 

 the rudiments of hind limbs that border the vent : these are best 

 seen in Boa, Python, Eryx, Tortrix ; it is developed into small 

 horns above the eyes in Vipera cerastes. In the Rattlesnake the 

 epiderm forms a series of hard moveable rings at the end of the 

 tail, twenty to thirty in number in full-grown specimens, 

 decreasing in size to the end of the series. The terminal (3 to 8) 

 caudal vertebra? coalesce into a long conical bone, covered by 

 thick, soft, vascular derm, divided by two deep annular grooves 

 into three transverse swellings : the basal ring of one joint grasps 

 the projecting second ring of the preceding joint, and this 

 incloses the third ring of the joint next but one in advance. 

 Since the second rounded annular portion of each joint is thus 

 securely grasped by the first rounded annular portion of the piece 

 behind it, and the third by the second, and yet all of them so 

 loosely as to leave room for motion, it has been supposed that 

 when the foremost piece has been completed, and a new piece in 

 advance is about to be formed, the skin which is to secrete it is so 

 modified that its first swelling, which secreted the first projection 

 of the former piece, assumes that shape and size which are 

 accommodated to the shape and size of the second projection of 

 the new piece, whilst the second swelling which secreted the 

 second projection of the piece takes the dimensions suited to 

 the third projection of the future new ring. The basal projections 

 of the successive rings are chiefly visible externally, only the first 



