OF THE IlfTEGUMEHTS. 
73 
the form of the trunk, varies according to the kind of 
locomotion which the animal performs, and that they are 
sometimes convex, at other times angular at the sides and 
plain below, and sometimes even sloping at the lateral 
angles. Their surface, entire in most Ophidians, is pro- 
vided with two protuberances in the Hydrophis properly 
so called, which probably arises from each of them being 
formed of two pieces soldered together. Several other 
species of the same genus present this remarkable charac- 
ter, that the middle line of the abdomen is indicated by 
a suture produced by a range of scales with which each of 
its sides is garnished ; lastly, the Acrochordus has a salient 
crest running along the abdomen, which is bristled with 
small mucronated scales : similar scales cover all parts of 
the body of this genus. 
The scaly plates below the tail form a single middle 
range in the Boa, the Eryx, and some other Ophidians. 
Most of the other animals of this order have that organ 
furnished with a double row of plates ; and it is from this 
circumstance that the term of Divided plates is derived, in 
opposition to simple plates or bands. 
The head of Ophidians is very rarely covered by scales 
resembling those on their bodies ; we there find seve- 
ral plates, larger than the rest, covering the different 
organs placed in the cranium. Most serpents even have 
the head fortified by plates of a more or less determinate 
and symmetrical form, always with a smooth surface, but 
the arrangement and form of which are subject to innu- 
merable modifications. As the forms of these scaly plates 
present characters easily comprehended, they have been 
employed to afford distinctive characters ; and this has 
given rise to a nomenclature, invented expressly to desig- 
nate these organs, according to the regions they occupy. 
The plates of the true Coluber being the most symmetrical 
in their disposition, have been regarded as of a normal 
form ; and all others appearing to be modelled on this type, 
it is easy to describe the numerous modifications which 
they undergo either by excess or default. The plates 
which invest the immoveable parts of the cranium, as 
those on the top of the headj possess a movement either 
