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Show 244 MR. BEDDARD ON DOLICHOTIS PATAGONICA. [Apr. 7, Lepus cuniculus, Sciurus vulgaris, Ccelogenys paca, Sphingurus prehensilis, Cavia porcellus, Lagostomus trichodactylus, and with the following Rodents known to me only by descriptions and figures :-Erethizon dorsatus (Mivart, " Notes on the Anatomy of Erethizon dorsalus," 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 ) ; Castor, 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 tbe 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 Ccelogenys are most like Dolichotis, but in the two former the furrow dividing the external gyrus is broken in the middle. In Ccelogenys the Sylvian fissure is hardly marked, but the rhinal fissure is exceedingly deep ; Ccelogenys 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. |