244 MR. BEDDARD ON DOLICHOTIS PATAGONICA. [Apr. 7, 



Lepus cuniculus, 

 Sciurus vulgaris^ 

 Coelogenys paca, 

 Sphingurus prehensilis, 

 Cavia porcellus, 

 Lagostomus trichodactylus, 



and with the following Eodents known to me only by descriptions 

 and figures : — Erethizon dorsatus (Mivart, " Notes on the Anatomy 

 of Erethizon dorsatus," P. Z. S, 1882, p. 250, woodcut fig. 7) ; 

 Pectinator spekii (Peters, " Contributions to the Knowledge of 

 Pectinator &c.," Trans. Z. S. vol. vii, p. 405, pi. 50. figs. 7-10) ; 

 Hystrix (Owen, ' Comp. Anat. of Vertebrates,' vol. iii. p. 110, 

 fig. 77, and Gratiolet and Leuret, Comp. Anat. Syst. nerv. pi. iii. 

 figs. 1, 2); C«*tor, Agouti, Paca, "Water-Rat, Squirrel, Rabbit (all 

 figured in the work just referred to). 



In comparing the brain of Dolichotis with those of the other 

 types mentioned in the above list, I have paid particular attention 

 to the classificatory importance of this structure. The primary 

 division of the Rodentia into Duplicidentati and Simplicidentati is 

 borne out by the characters of the brain. In the brain of the 

 Rabbit the lateral lobes of the cerebellum form an angle with the 

 middle lobe and reach rather further forward, while the flocculi 

 stand out prominently. The rhinencephalon is sharply bent down, and 

 the posterior half forms an exceedingly prominent convex projection. 

 These characters are not met with in the other genera that I have 

 examined. At the same time I cannot find any characters except 

 negative ones that bind together the Simplicidentati. 



Lagostomus and Coelogenys are most like Dolichotis, but in the 

 two former the furrow dividing the external gyrus is broken in the 

 middle. In Coelogenys the Sylvian fissure is hardly marked, but the 

 rhinal fissure is exceedingly deep ; Coelogenys shows a further point 

 of agreement with Dolichotis in the presence of a supraorbital sulcus ; 

 this extends further back, running parallel with the rhinal fissure in 

 Dolichotis. The most important fissure in the pallium of the 

 Rodent's brain is clearly the one which bounds the external gyrus. 

 In the Rabbit and Guinea-pig, which have nearly smooth brains, 

 traces of this furrow exist for a short extent posteriorly. Probably 

 the dints on the upper surface of the brain in the Porcupines are 

 still further reduced traces of the same furrow. I may take this 

 opportunity of mentioning that the brain of Sphingurus prehensilis 

 agrees very closely with Mivart's description and figures of the 

 brain of Erethizon dorsatus. I cannot indeed detect any point in 

 which they differ. 



On the whole it seems that the anatomy of Dolichotis brings it 

 into relation with the Agoutis rather than the Porcupines. 



