1908.] BATRACHIAN KHINODERMA DARWINI. 687 



These two muscles appear to me to be quite jDossibly the genio- 

 glossi, which are thus in the species Rhinoderina darwini not only 

 of very lai'ge size but rather abnormal in position. They are 

 normal only in that they arise on either side from the mandible ; 

 they are abnormal in that they have intruded into an area 

 belonging to the submaxillaris which ceases to exist as a separate 

 layer at the margin of the genioglossi. Furthermore, the intru- 

 sion of the genioglossi on to the superficial area of the throat has 

 caused another peculiarity in the arrangement of the fibres of the 

 submaxillaris. It will be seen from an inspection of text-fig. 146 

 that the fibres of the submaxillaris run in difierent directions in 

 different portions of this muscle. Laterally to the possible genio- 

 glossi the fibres of the submaxillai'is run obliquely to each ramus 

 of the lower jaw ; posteriorly to them the fibres are at right angles 

 to the longitudinal axis of the head and run therefore accurately 

 across the throat, with no obliquity of direction like the anterior 

 part of the muscles. If I am right in identifying the triangular 

 pair of muscles just described with the genioglossi of other Batra- 

 chians, they certainly difier in not being inserted on to the tongue 

 (which of course does not necessarily do away with this suggested 

 homology), for they can be easily dissected away with the sub- 

 maxillaris, displaying the hyogiossus and geniohyoidei beneath. 

 In any case, whatever be the nature of these muscles, it is clear 

 that the muscular floor of the mouth in Rhinoderma darwini is 

 peculiar and unlike that of other frogs. I should mention, 

 furthermore, that there is no vestige in this frog of the small 

 muscles at the side of the mouth lettered " x " in my figure of 

 the muscular system of the ventral surface of the body in 

 Breviceps verrucosus *. It may bei that these muscles are 

 the homologues of the peculiar muscles which I describe in 

 Rhinoderma (text-fig, 146, a). If so, they have undergone in the 

 one or in the other genus a very considerable shifting of position. 

 It must be borne in mind that the species the anatomy of which 

 forms the subject of my present communication to the Society 

 is hax'dly to be described as " engystomatous." 



§ The Afuscidature of the Back. 



The latissimus dorsi (text-fig.147, L.d.) is distinctly difierent from 

 that of Rana, as figured. It is a distinctly narrow muscle, being 

 about one half of the diameter of the underlying infraspinatus. 

 Its course is straight and is exactly at right angles to the long axis 

 of the body. It has no obliquity of direction as in other frogs. 

 It is therefore also exactly parallel to the partly underlying infra- 

 spinatus. It arises from the middle line of the back behind the 

 scapula. It did not appear to me to arise from the dorsal fascia ; 

 but as this pigmented membrane was so delicate it had to be 

 picked away in little bits, and its relations were therefore rather 



* P. Z. S. 1908, p. 16, text-fig-. 3. 



Proc. Zool. Soc— 1908, No. XLIY. 44 



