682 CASTORIDAE—TROGONTHERIUM 
recently Sir E. G. Loder has established a beaver-pond on_ his 
estate at Horsham, Sussex (?.Z.S., 1898, 201), and this colony is 
still thriving. 
[GENUS TROGONTHERIUM. 
1809. TROGONTHERIUM, G. Fischer von Waldheim, Jem. and Soc. Imp. Nat. 
Moscow, ii., 260; Owen ; Newton. 
1823. Castor, G. Cuvier (in part); 1848, Diabroticus, A. Pomel; 1862, Conodontes, 
Laugel. 
TROGONTHERIUM CUVIERY, Fischer. 
1823. TROGONTHERIUM CUVIERI, G. Fischer in Cuvier, Rech. Oss. Foss., ed. 2, v., 
59, based ona 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, Bzblio. Univ. Gendve Arch. Sci., ix., 
167. 
1862. CONODONTES BOISVILLETTEI, Laugel, Su//. Soc. Géol. Fr. (2), xix., 709. 
The largest British rodent, although long extinct, deserves a brief 
notice here. Fischer based his genus 7vogontherzum 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 Zvogontherzum 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 Cas¢or ; 
m and m* are small, while 7* 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 Castory chiefly in that the ventral 
surface of the basi-occipital shows the normal median ridge and shallow 
lateral fossze instead of the peculiar deep pharyngeal pit of the Beaver ; 
1 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). 
