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Show 1882.] PROF. F. J. BELL ON THE GENUS PSOLUS. 641 ing features of the Dipodida connect them with the Hystricomorpha, as, for example, the large infraorbital foramen (rivalling even the orbital in size) and the stout zygomatic arch in which the malar is not supported by a continuation of the maxillary zygomatic process- characters eminently distinctive of the hystricine rodents. Moreover, as in this group, none of the molars are tuberculate, but exhibit transverse laminae as in Chinchillida, with the species of which family the Dipodida agree, not only in many striking superficial points of resemblance, as in the shape of the ears, muzzle, &c, but also in the peculiar form of the penis, of which the glans is armed, as in the Cavies and Pacas, on the upper surface with a pair of soft spines and numerous horny scutes, so differing essentially from the soft unarmed state of the same part in the Myomorpha. The united condition of the leg-bones is evidently the result of special adaptation of the hind limbs for leaping ; and it would be as absurd to separate this family from the Hystricomorpha, on this account, as it would be to elevate the Dipodina into the rank of a distinct family, and form a new group for their reception, because they differ from all other rodents in the united condition of the metatarsals, which are fused together so as to form a single bone, a condition as manifestly the result of adaptive modification as the union of the fibula with the tibia. W e may conclude, therefore, that the Dipodida must be classed as hystricine rodents having the bones of their hind limbs specially modified for leaping, and that their nearest existing allies are the family Chinchillida. 4. Studies in the Holothuroidea.-I. On the Genus Psolus and the Forms allied thereto. By F. J E F F R E Y B E L L, M.A., F.Z.S., Professor of Comparative Anatomy in King's College, London. [Received October 18, 1882.] (Plate XLVIII.) In the following paper, and in those of which it will as I hope the first, it is m y intention to bring together into a connected form all the information which I have acquired in the difficult task of naming the collection of Holothurians in the British Museum. Various circumstances did no doubt conspire to prevent these specimens being worked out as they came into the collection ; but I fancy I am hardly wrong in imagining that a not unconsidered factor was the troublesomeness of the subject, and the great demand that it makes upon the time and patience of the student. Works of the highest importance and greatest scientific value have appeared on these forms ; the anatomical monograph of Tiedemann, the researches of Johannes Miiller, and the magnificent firstfruits 43* |