384 Orbicular Lizards, or Horned Agamas. 
more barren parts of the country. Hitherto great confusion has ex 
_ isted among authors relative to the animals classed under this name. 
The Orbicular lizard of Linneus and Agama orbicularis of Daudan 
and Tapayaxin of Seba, belong to an entirely different genus, the 
Trapelus, which inhabits Africa: these facts were first announced in 
a memoir which I published in the year 1824, vid. Journ. A. N.S. 
Philad. Vol. iv. pl. xx,—on the Agama cornuta, of Missouri and 
Arkansas, and other portions of the United States territories. A 
very critical essay on these Orbicular lizards was published by Gra- 
venhorst in 1833, vid. Acta Acad. Caes, Leop. Carol. Nat. Cur. 
Vol. xvi. p. ii; in which the learned author has fallen into some 
errors. Together with Cuvier and Wagler, he confounds the Tapa- 
yaxin of Hernandez with the Agama cornuta of Harlan, whereas 
these form two very distinct species: he further errs in attributing 
to Weigman the priority in contributing any definite information 
relative to this animal, as is shown by the following extract from the 
memoir above quoted: ‘‘ Weigman of Berlin was the first, as far as 
I know, who gave any definite information concerning this animal, 
and who showed at the same time that it differed from the others 
with which it had been compared, not only in the species, but also 
in the genus.” p. 912. None of the naturalists above quoted ap- 
pear to have had an opportunity of comparing our Agama cornuta 
with the Phrynosoma orbicularis of Weigmann, otherwise they could 
scarcely have overlooked the prominent characters which distin- 
guished these two species: there can be no doubt, on accurate com- 
parison, that the last named animal is the true Tapayaxin described 
and figured by Hernandez ; and which differs in size, markings and 
proportional development of the tail, from the Agama cornuta. We 
have received numerous specimens of the Agama cornuta from 
Mexico, as well as from the United States and territories, the largest 
of which are little more than one half the size of the Tapayaxin ; 
the horns of the head also differ in their proportional size, and some- 
what in their arrangement; the large transverse black bands on the 
back are peculiar to the Phrynosoma or Agama orbicularis: whilst 
in the A. cornuta there is also a constriction at the base of the tail, - 
_which is absent in the former,—the most accurate figure of the A. 
orbicularis is given by Gravenhorst. The occipital spines in these 
animals partake of the nature of true horns, consisting of an osseous 
core with a sheath of horn, which becomes easily detached in ma- 
cération. 
