OF TU£ INTEGUMEN.TS. 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 Hydropliis 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 b3 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 head, possess a movement either 



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