NATURAL SCIENCES OF PHILADELPHIA. 75 



Head without markings, except a darker shade posterior to the eye. Rela- 

 tions of vertical and occipital plates as in typical v u d i i. 



In the extensive series of serpents possessing the diacranterian dentition and 

 bifid anal scutum, which connect the stout, heavy bodied Xenodons with the 

 slender Drymobii, the authors of the Erpetologie Generate have recognized 

 two generic forms, viz. Liophis, Wagl., and Dromicus, Bibr. These they sepa- 

 rate upon a difference in the relative development of the tail ; in the former 

 this member is said to be short, in the latter elongate. In thus defining their 

 groups thev have well contrasted the prominent characteristics of the extremes 

 of the series in question. Another point of contrast is here added, viz. in the 

 short tailed extreme there are no scale pores ; in the longest tailed, these pores 

 exist in pairs. The first may be represented by the Liophis c o b e 1 1 a, the 

 last, by the Dromicus a t e r of authors. That these species belong to dif- 

 ferent genera admits of no doubt; they are placed in different "families" by 

 some authors. The Dromicus a t e r, and its immediate allies, more nearly 

 resemble in general form and habits some species of Drymobius, than they do 

 the Liophis c o b e 1 1 a ; the latter represents a genus of water snakes, the 

 former are terrestrial and arborial. 



The relative length of the tail cannot be entirely relied upon as a definite 

 index of the genera included between these extremes. There are species in 

 which this member is of an intermediate length, and some of these though 

 included by authors in their genus Dromicus, can by no means be separated 

 from the genus Liophis, as has been elsewhere shown.* From these types to 

 that of the D. a t e r, the gradation seems complete. In like manner the num- 

 ber of rows of scales on the body is a safe index of genera in some parts of 

 the system, in others it is not specific, varying with the age and circumstance of 

 the individual ; the same may be said of the division of the anal plate and pre- 

 ocular, of the carination of the scales, of the grooving of posterior maxillary 

 teeth ; also of the number of the toes in the sauria, and in an infinite number 

 of instances which will occur to every zoologist. Where, however, an organ 

 exhibits a perfect gradation between its different type forms, as we know to be 

 the case with most or all, at one or more points in the morphic scale of each, 

 the usual breaks or steps in this scale of modification of some other structure 

 or organ, most commonly indicate to us Nature's divisions as at present exist- 

 ing. 



In passing from Dromicus a t e r toward the species with shorter tails, we 

 find the two scale pores become reduced to one. Finally, in the third 

 series, typified by Dromicus lineatus, where the pores have disappeared, 

 the tail never, so far as is known, equals that of the Dromicus ater 

 group in length, nor is it thick, nor is the body heavy as in the poreless 

 cob ell a group. A fifth series, also with poreless scales, represented by 

 Dromicus fugitivus, exhibits the very elongate tail of the ater group. I 

 know of no species connecting it with the third, or lineatus type, though 

 the discovery of such an one would not be a matter of surprise ; in that case 

 the forms would be included under one head. The groups thus defined, with 

 their species, may be enumerated as follows : 



Ophiomorphus Fitz. (sine diagnosi). Body short, stout. Head distinct. 

 Scales poreless. Tail short, thick, about one-fifth the total length. (Liophis 

 Fitz. Dum.) 



0. co b el la, 0. merr emm ii,f (type,) 0. d o li a t us, 0. breviceps. 



Lygophis Fitz. (sine diagnosi). Body, slender, elongate. Scales poreless. 

 Tail one-fourth the total length, sometimes a little longer, rarely shorter, 

 always slender. 



* Proc. Acad. Nat. Sci., Phila., 1860, p. 251. 



t Xenodon typhlus, Schl. is almost identical in form with these species, but our speci- 

 mens being wiihout epidermis, the absence or presence of pores cannot be determined. 



1862.] 



