1882.] PROF. F. J. BELL ON THE GENUS PSOLUS. 641 
ing features of the Dipodide 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 lamine as in Chinchillide, with the species of which 
family the Dipodide 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 Dipodine 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 siugle 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 Dipodide 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 Chinchillide. 
4. Studies in the Holothuroidea—I. On the Genus Psolus 
and the Forms allied thereto. By F. Jerrrey Bett, 
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 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 
Iam 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* 
