CHARACTERS OF SOME HYSTRICOMORPH RODENTS. 421 



subservient to aquatic life. It is used mainly as a rudder, hardly 

 as a propeller, in swimming. 



In the Hystricidae the tail is always pi-ovided with a sounding- 

 organ formed of modified hairs. In the most primitive type 

 Trichys, in which the ai-mature of the body-skin consists of a 

 coating of very coarse sharp bristles, nearly uniform in length, the 

 tail is long and cylindrical, and covei-ed, except at the root and 

 tip, with scales and short hairs ; and the sounding-organ or rattle 

 at the tip consists of a brush of long flattened hairs or spines 

 somewhat resembling dried blades of grass. In Atherura the 

 tail, although still scaly and hairy, is much shorter, and each of 

 the constituent parts of the terminal brush consists of a filiform 

 axis expanding along its course into a series of compressed but 

 hollow fusiform swellings, from two to seven or eight in number, 

 according to the length of the blade. These swellings vary in 

 size, but the one at the end is always much larger than the next 

 of the series. 



In Thecurus, Acanthion, and Hystrix, in which the spine- 

 armature reaches its maximum of development, the tail is quite 

 short and thick and without scales and hairs ; but twenty or 

 more of its terminal quills are expanded into hollow flattened 

 laminae or cylinders, for the most part open at the end. This is 

 the most highly specialised rattle in the group. 



The so-called arboreal Porcupines of America (Erethizontidse) 

 fall into two sharply defined groups by their tails. In the North 

 American genus Erethizon this organ is quite short and armed 

 throughout with spines resembling those on the body. By 

 swinging it to I'ight and left, the animal uses it as a weapon of 

 defence. In the tropical American genera Coendu and Chcetomys 

 the tail is long, siabcylindrical, tapering, and covered with spines 

 and hairs and clistally with scales, although the extremity, which is 

 upcurled and prehensile, is naked above. The underside at the 

 base is thickly covered with close-set, stiff, sharp bristles, the 

 function of which, as Waterhouse rightly supposed, is to help 

 in the ascent of vertical or steeply sloping branches, and to give 

 support to the body when the animal is at rest. They are func- 

 tionally comparable to the caudal scales of the Anomaluridae. 



In the three genera assigned to the Chinchillidse, namely 

 Chinchilla, Lagiclium, and Lagostomus, the tail is also charac- 

 teristic. It is moderately long, covered with hairs, which are 

 comparatively short and soft all along the underside, but on 

 the upperside are coarse and very long — forming, as it were, 

 a brush. 



In the Dasyproctida3 and Caviidse the tail is reduced. In 

 Myoprocta it is at most a few inches long, but quite slender. 

 In Dasyprocta it is only about an inch or less. In Ocelogenys 

 it is about the same. In Dolichotis it is also quite short, but 

 constricted at the base and oval in outline from above or below. 

 In Hydrochcerus it is at most a short conical excrescence as it is 

 in Gavia, Gcdea, and Kerodon, and is sometimes absent. 



