1882.] PROF. F. J. BELL ON THE GENUS PSOLUS. 641 



ing features of tbe Dipodidce connect them with the Hystncomorpha, 

 as, for example, the large infraorbital foramen (rivalling even the 

 orbital in size) and the stout zygomatic arch in wliich 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 laminse as in Chinchillidce, with the species of which 

 family the Dipodidce 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, ou this 

 account, as it would be to elevate the Dipodince 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. 



We 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 Chinchillidce. 



4. Studies in the Holotliuroidea. — I. On the Genus Psoitis 

 and the Forms allied thereto. By F. Jeffrey Bell, 

 M.A., F.Z.S., Professor of Comparative Anatomy in 

 King's College, London. 



[Eeceived October 18, 1882.] 

 (Plate XL VIII.) 



In the following paper, and in those of which it will as I hope form 

 the first, it is my 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 speci- 

 mens 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 



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