74 ON THE PHYSIOGNOMY OF SEHPENTS. 



very limited or null. The skin Avhich enters into their 

 composition is very thin, and most frequently glued to the 

 cranimn. A pairless plate is very generally observed on 

 the summit of the head, called The Vertical, which may 

 be said to present an innnoveable centre, around which 

 the other scaly plates are arranged : it is generally pen- 

 tagonal, with the base toward the muzzle ; it is sometimes 

 very narrow, at other times very vvide, according to 

 the general form of the head ; and it affects a trigonal, 

 an hexagonal, or a lanceolate form, according to the na- 

 ture of the surrounding plates ; it has an irregular shape 

 in several of the genus Boa, or is even divided into 

 two pieces by a longitudinal suture : in other Ophidians 

 its volume is so reduced, that it ceases to be distinguish- 

 able from the other scales. 



That ]ilate is often followed by a pair termed The Oc- 

 cipital, plates of somewhat a trajiezoidal form, but very 

 variable in the different species : these plates are always 

 in contact at their inner edges ; and it is only in the Tor- 

 trix scytale and the T. Xenopeltis that they occupy the 

 sides of the head, and receive between them a middle 

 supernumerary plate, which resembles the scales of the 

 trunk. The occipitals never exist without the vertical : 

 they are very small in several species of the genera Dipsas, 

 Xenodon, Ilomalopsis, Hydrophis, TortrLx, Boa, &c. ; we 

 merely find vestiges of them in the Trigonocephalus Cen- 

 chris, and they are replaced by some little plates of a very 

 irregular form in some of the Boas. 



The Superciliary are a pair of plates placed at the sides 

 of the vertical, and protecting the eye from above ; they 

 almost always run along the orbit, forming a vault very 

 little moveable, under which the globe of the eye can 

 freely exercise the limited movements which it enjoys. 

 Their form and their extent vary infinitely : sometimes 

 convex, sometimes hollowed at their external edge, most 

 generally vaulted, and sometimes fiat, they are raised 

 up in the Acanthophis, while their surface in the other 

 Ophidians is in the same plane as the top of the head. 

 They are placed far back in several of the genus Tortrix, 

 and are united firmly to the single ocular plate in the 



