RODENTIA. 379 



are still allusions to it in some names of places (e.g., Beverley, 

 Nant-yr-afancwm). Numerous skeletons are found in the valley 

 of the Lea, Essex, and in the fens of the eastern counties. A 

 very large extinct beaver, Trogontherium, also ranged through- 

 out Europe in the early Pleistocene ; the finest known skull 

 of this animal, about 016 m. in length, was obtained from the 

 Forest Bed of Cromer, Norfolk. 



Some of the Eocene and Miocene rodents, as also the sur- 

 viving African family of Anomaluridae, are intermediate 

 between the Sciuromorpha and the porcupine-like group, or 

 HYSTRICOMORPHA. In the latter, as a rule, the zygomatic arch 

 is massive and the infraorbital foramen is much enlarged, while 

 the angular part of the mandible does not arise from the 

 inferior surface of the socket of the lower incisor, but from its 

 outer face. The intermediate forms just mentioned exhibit 

 the Sciuromorph mandible and zygomatic arch, with the much- 

 enlarged infraorbital foramen. Theridomys, Sciuroides and 

 Pseudosdurus, from the Upper Eocene of France, England, 

 Germany, and Switzerland, may be mentioned as European 

 examples; while Ischyromys, from the Lower Miocene (White 

 River Formation) of Colorado, is perhaps the best-known 

 American genus. The true Hystricomorpha have always 

 flourished most in South America, where they still exhibit an 

 extraordinary development and comprise the largest known 

 members of the Rodentia. The extinct Megamys, from the 

 lower beds of the Pampa Formation of the Argentine Republic, 

 is estimated to have been nearly as large as an ox ; while the 

 surviving capybara (Hydrochcerus capybara) attains a length of 

 over a metre. An equally gigantic rodent, Castoroides ohioticus, 

 which seems to be Hystricomorph in most respects, is known 

 by the complete skeleton from Indiana and by numerous frag- 

 mentary remains from other parts of North America. Its 

 upper incisors are not completely scalpriform, but nearly sur- 

 rounded with enamel. The true porcupines (Hystrix) are now 

 confined to the Old World, but skeletal fragments from the 

 Miocene and Pliocene of North America have been provision- 

 ally ascribed to the same genus. 



The rats, mice, and their allies, or MYOMORPHA, seem to be 



