404 



OBSEBV. This species, together with P. coronatum, is one of the 

 largest of the group. It is also the one which is most commonly 

 known, since it inhabits a more extensive geographic range than its 

 congeners, as also regions so far more accessible to travellers and ex- 

 plorers. The recent settlement of California, however, has been 

 instrumental in rendering P. coronatum quite common in our col- 

 lections. 



DESCR. The spines of the head are very conspicuously developed. 

 There is a pair of occipitals, the largest of them all ; exteriorly to which, 

 three temporal ones may be observed, diminishing in size outwardly ; 

 finally, there is a fifth above the posterior rirn of the eye, and about 

 the size of the external occipital pair. From the supraocular spine 

 forwards, a thick ridge (the supraciliary ridge), extends almost to the 

 margin of the jaw, inclosing a subcordiform area, which is sloping 

 towards the nostrils : the latter being situated within the said ridge. 

 The snout is rather abruptly sloping. The labial plates are exceed- 

 ingly small, and scarcely distinguishable from the adjoining supra- and 

 infralabials, except the posterior lower labials, which are somewhat 

 larger. The mento-submaxillar shields, on the other hand, consti- 

 tute a very conspicuous series, quite sharp upon their outer edge, in- 

 creasing gradually in size backwards : the posterior one, spine-like, 

 lying beneath the auricular aperture. The upper surface of the head 

 is covered with small, polygonal, reticulated, or multicarinated plates, 

 largest upon the temporal and occipital regions, and smallest about 

 the orbits and sides of the head. The surface of the eyelid is covered 

 with small and smooth plates, whilst its margin is provided with 

 keeled and somewhat larger plates. The scales at the inferior surface 

 of the head are very small, slightly keeled, equal-sized, except a longi- 

 tudinal row on each side, in which they are somewhat larger, pyra- 

 midal, slightly raised, and directed outwards and backwards. The 

 auricular aperture is moderate, vertically oblong, and somewhat hid- 

 den in the midst of the numerous folds of the neck. 



The body above is densely covered with scales of various sizes, very 

 minute on the neck and sides of the abdomen, somewhat larger and 

 irregular on the back and tail, where the most conspicuous may be 

 observed, all having a tendency to be keeled. Scattered in the midst 

 of these, may be seen much, larger scales, of a pyramidal shape, 

 raised, prickle-like, above the general surface of the body ; they con- 

 stitute a double series at the periphery of the abdomen, between 



