74 
ON THE PHYSIOGNOMY OP SEEPENTS. 
very limited or nnlL The skin which enters into their 
composition is very thin, and most frequently glued to the 
cranium. A pairless plate is very generally observed on 
the summit of the head, called The Vertical^ which may 
be said to present an immoveable 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 wide, 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 plate is often followed by a pair termed The Oc- 
cipital^ plates of somewhat a trapezoidal 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, Homalopsis, Hydrophis, Tortrix, 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 flat, 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; 
