INTRODUCTION. 



EXPLANATION OF TERMS USED. 



The vertical plate is the central one in the middle of the head 

 above, having on each side of it the sxqierciliaries, which form the 

 upper part of the orbit. The two plates behind the vertical are the 

 occipitals; the pair in front of it, the postfrontols. The prefrontals 

 or anterior frontaU are situated in front of the postfrontals ; and an- 

 terior to these and terminating the snout is the rostral. The plates 

 immediately in front of the eye are the anteorhitals ; those behind it 

 are the postorhitals. In advance of the anteorbital is the loral, be- 

 tween which and the rostral are the two nasals, with the nostril be- 

 tween them. The iipper and lower labials margin the upper and 

 lower jaws. The temporal shields are situated between the upper 

 labials and the occipitals. The inframaxillary or mental scutellte 

 or shields are just within the lower labials. 



The arrangement on the top of the head of one rostral, two pairs 

 of frontals, one vertical with one superciliary on each side, and one 

 pair of occipitals we have considered as typical or normal, from which 

 but few of the genera described vary. Sometimes one plate occupies 

 the place of the two prefrontals, and in some genera a second median 

 plate is seen between the rostral, frontals, and vertical. On the side 

 of the head we have sometimes but one nasal, and sometimes either 

 the loral or the anteorhitals may be wanting. Where the latter con- 

 dition exists, it is sometimes diificult at first to determine which plate 

 has disappeared. A clue is to be found in the shape of the remain- 

 ing plate ; if this be longitudinal, it is probably the loral ; if vertical, 

 or divided into two or more, one above the other, it is to be con- 

 sidered as anteorbital. The loral belongs to the postfrontals, and 

 the anteorbital to the vertical, the posterior edges in the former and 



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