La cert a. 29' 



This ^enus is one ou whicli more atteution has been bestowed than 

 on any other among the Laeertilia, with astonishing differences of 

 opinion as to the delimitation of the species and their classification. 



I have recently (Tr. Zool. Soc. xxi, 1916) explained the principles 

 which have guided me in the systematic arrangement, and my remarks 

 are here reproduced. 



It is not often, when having to deal with the phylogeny of existing 

 species, that one can point to any, actually living at the present time, 

 as forming part of the probable ancestral stock ; and yet, in this case, 

 I feel pretty confident that L. agilis and its close ally L. parva have 

 preserved the primitive characters out of which the series represented 

 in Sections I, III and IV of the following classification have been 

 evolved. L. agilis is a widely distributed species, now ranging over 

 the greater part of Europe and a considerable part of northern and 

 temperate Asia ; it is highly variable both in its lepidosis and in its 

 markings, and, even without imagining a greater amplitude of varia- 

 tion than is known in the existing individuals, we find in it a combina- 

 tion of characters which realise the ideal ai*chaic type leading, through 

 more or less broken chains of forms, still in existence, to the most 

 extreme modifications to be found in the three groups mentioned. 



So far as the very scanty palaeontological material allows us to judge, 

 the genera Lacerta and Nucras are the only representatives of the 

 family Lacertidse known to occur as far back as the Upper Eocene and 

 the Oligoceue,* and these two, which are intimately connected and 

 barely separable, must be looked upon as the original ancestral types, 

 out of which the allied genera Psammodronms, Latastia, Acantlio- 

 dactylus, Cabrita, Ophiops, Ereniias, etc., have been derived by a 

 series of modifications which may be formulated as follows : 



1. Reduction and disappearance of the teeth on the palate. 



2. Flattening and weaker ossification of the skull (reduction of the 

 postfx'onto- squamosal arch), together with elongation and acumination 

 of the rostrum, accompanied by approximation of the nares to each 

 other (reduction in width of the ascending process of the pre- 

 maxillary) ; reduction in some series, increase in others, of the 

 osteo-dermal plates. 



3. Disappeai'ance of the foramen parietale. 



4. Disintegration of the head-shields : division of the elements 

 surrounding the nostril ; intercalation of granules around the supra- 

 ocular shields ; multiplication of the temporal and labial shields ; 



* Filbol, Ann. Sci. Geol. viii, 1877, p. 269; De Stefano, Atti Soc. Ital. Sc. 

 Nat. xlii, 1901, p. 412 ; Klebs, Schrift. Phys.-cek. Ges. Konigsberg, li. 1910, 

 p. 227. 



