340 NATURAL SCIENCE. Nov.. 



quite possible to place a specimen in its proper family by referring to 

 the combinations of muscles which are characteristic of that family. 

 For instance, the absence of a scalenus anticus in a hystricomorphine 

 rodent would at once make me suspect that it was a porcupine ; if it 

 wanted a peroneus quarti I should suspect it of being a tree- 

 porcupine ; and if in addition it had two heads to the biceps cubiti, 

 a well-developed omo-hyoid, and a levator claviculae rising from the 

 atlas, I should feel pretty certain that it was one. 



The question which I should expect to be asked, and which, 

 indeed, has been asked, is " What light does myology throw on the 

 position of the Dipodidae ? " Dobson' says that the only argument 

 for placing them among the Myomorpha is the fact that the tibia and 

 fibula are fused, while in favour of including them in the Hystrico- 

 morpha are the united flexors in the sole, the masseter passing 

 through the infraorbital foramen, the external appearance of the ears 

 and muzzle, the armed condition of the penis, and the arrangement of 

 the teeth. 



With regard to the fused tendons, I have been able to point out 

 two examples of myomorphine rodents in which these are present. 

 The large size of the infraorbital foramen is a question of degree, 

 since in most myomorphine rodents a small piece of the masseter 

 passes through this opening, and it is only in sciuromorphine and 

 lagomorphine rodents that the infraorbital foramen transmits nothing 

 but the nerve. With regard to the classificatory value of teeth, 

 Mivart, in his work on the iEluroidea,^ has given grounds for not 

 placing much confidence in them, and, for my own part, I cannot 

 help thinking that, unless used with considerable caution, they are 

 apt to mislead. I can add another claim to those which Dobson has 

 given for regarding the jerboas as hystricomorphine, and that is that 

 they have only one head to the biceps cubiti, while every myo- 

 morphine rodent that I have looked at possesses two. On the other 

 hand, in addition to the fusion of the leg-bones, which is never seen in 

 the Hystricomorpha, the two halves of the lower jaw move upon one 

 another and are provided with a transverse mandibular muscle ; the 

 digastric is arranged on the sciuromorphine type described by 

 Kunstler,3 a type which is never found in the Hystricomorpha, but 

 often in the Myomorpha ; the scapulo-clavicularis, which I have 

 already laid stress on as being a most constant muscle in the Hystri- 

 comorpha, is absent ; and the omo-hyoid is present as in all the 

 Myomorpha, while it is often absent in the Hystricomorpha. On the 

 whole, I certainly think that the myology of the jerboas points to 

 their having myomorphine rather than hystricomorphine tendencies, 

 though their many points of difference from both groups might entitle 

 tliem to subordinal rank, as Dipodomorpha. With regard to the 

 affinities that Dobson believes them to have with the Chinchillidie, a 



^Proc. Zool. Soc, 1882, p. 640. ^Pioc. Zool. Soc, 18S2. 



^ Ann. Set. Nat., ser. 7, t. iv., p. 150. 



