10 INTRODUCTION. 



others, it approaches R. esculenta. In colouring, R. oxyrhinus re- 

 sembles R, temporaria ; the throat, however, is usually pure white, 

 at least in the males, the breast dusky white and spotted, while in 

 R. temporaria the throat and breast are more uniformly coloured 

 and yellowish. V. Siebold has remarked that, during the pairing- 

 season, the males are covered with a bluish bloom 1 ; and, the 

 whole ground colour being bright at this period, very beautiful 

 tints result. V. Siebold 2 moreover states that the note which the 

 males produce during the pairing-season is different in the two 

 species. On the whole, R. oxyrhinus appears to stand midway 

 between R. esculenta and R. temporaria. 



Thomas 3 , in addition, distinguishes another species, R. agilix, 

 which however may be the ' grenouille rousse' of Millet. Schlott- 

 hauber 4 has described a frog which, in marking and colouring, might 

 hold a middle place between R. esculenta and R. temporaria ; in my 

 opinion this is probably a cross between the two. That attempts at 

 copulation are made, despite the difference of the pairing-season, is 

 well known ; Pontallie 5 mentions this, and I have myself often found 

 males of R. temporaria in conjunction with females of R. esculenta. 



I use the following terminology. I suppose the animal to be in 

 its natural position, the belly towards the ground, the back up- 

 wards; a horizontal plane passing from the snout to the anus 

 divides the body into a superior or dorsal half and an inferior or 

 ventral half. The terms superior and inferior, dorsal and ventral, 

 indicate positions with relation to this plane. I call that part 

 anterior which looks towards the head, and that posterior which 

 looks towards the anus. A vertical plane at right angles to the 

 middle of the longitudinal axis of the body, divides it into an 

 anterior or cephalic and a posterior or caudal half. All sections 

 and planes which lie parallel to this, as well as this itself, are frontal. 

 Lastly, by a perpendicular section along the middle line of the body 

 the animal is divided into right and left halves; this plane is the 

 median plane ; and the position relative to this plane is expressed 

 by the terms median or lateral. Planes parallel to the median plane 

 are termed sagittal. 



1 I do not find, however, that this disappears when the animals are on land ; in 

 fact I have animals before me in a glass in which it is plainly seen. 



2 I.e., p. 15. 



3 Annales des Sciences naturelles. Zoologie, IV Se"rie, Vol. IV, 1855. 



4 Wiegmann's Archiv, Vol. 1, 1844, p. 255. 



5 Annales des Sciences naturelles. Zoologie, III Se"rie, Vol. XVIII, 1852, p. 243. 



