682 CASTORID^— TROGONTHERIUM 



recently Sir E. G. Loder has established a beaver-pond on his 

 estate at Horsham, Sussex {P.Z.S., 1898, 201), and this colony is 

 still thriving.^ 



[Genus TROGONTHERIUM. 



1809. TROGONTHERIUM, G. Fischer von Waldheim, Mem. and Soc. Imp. Nat. 



Moscow, ii., 260 ; Owen ; Newton. 

 1823. Castor, G. Cuvier (in part) ; 1848, Diabroticus, A. Pomel ; 1862, Conodontes, 



Laugel. 



TROGONTHERIUM CUVIERI, Fischer. 



1823. TROGONTHERIUM CUVIERI, G. Fischer in Cuvier, Rech. Oss. Foss., ed. 2, v., 

 59, based on a skull from sandy deposits of uncertain age near the Sea of Azof ; 

 Owen ; Newton. 



1823. Castor trogontherium, G. Cuvier, Rech. Oss. Foss., ed. 2, v., 60. 



1848. Diabroticus schmerlingi, A. Pomel, Biblio. Univ. Genive Arch. Set., ix., 

 167. 



1862. Conodontes boisvillettei, Laugel, Bull. Soc. GM. Fr. (2), xix., 709. 



The largest British rodent, although long extinct, deserves a brief 

 notice here. Fischer based his genus Trogontherium upon a fossil skull 

 found in a sandy deposit exposed near the Sea of Azof. On the basis 

 of drawings of this skull, sent to him by Fischer, Cuvier was unable 

 to appreciate any generic distinction from Castor, although he estimated 

 Trogontherium to have been fully one-fifth larger than the largest living 

 Beaver. The subsequent discovery of fine material in English strata 

 (and its able description by Owen and Newton) leaves no room for 

 doubting the title of Trogontherium to full generic rank. 



The dental formula is as in Castor. The incisors are much larger 

 and less strongly curved. The cheek-teeth have triangular instead of 

 squarish crowns, and they develop roots at a comparatively early age. 

 The premolars are the largest, and are relatively larger than in Castor ; 

 m^ and m^ are small, while m^ is somewhat larger, and when little worn 

 is more complex. In the Beaver the cheek-teeth decrease in size 

 progressively from before backwards. While the enamel pattern is 

 essentially similar to that of Castor, the infolds soon lose their connec- 

 tion with the periphery of the tooth and become reduced to " islets " at 

 relatively early stages of wear. The vertical extension of certain of the 

 folds also is less, and they are soon entirely worn out. 



The skull differs from that of Castor chiefly in that the ventral 

 surface of the basi-occipital shows the normal median ridge and shallow 

 lateral fossae instead of the peculiar deep pharyngeal pit of the Beaver ; 



' Sir E. G. Loder told Cocks (Jan. 1917) that his Beavers have ceased to breed 

 for several years, but he has now introduced a young pair from the Zoo (keeping 

 them carefully separate, however, from the old ones). 



