OBSERVATIONS ON THE RODENTIA. 595 



phibia, and Ondatra {a) ; in Lemmus Norvegicus and many 

 of the smaller species of Arvicola this portion is rounded ; 

 in the remaining three genera, Geomys, Spalax and Castor, 

 the temporal bone is less produced ; it nevertheless encroach- 

 es considerably on the temporal fossce in these genera. The 

 superior maxillary bone sends backwards a lamellar process 

 (b in the figures), in most of the species of the present family, 

 as in the Muridas ; Castor and Geomys, however, afford ex- 

 ceptions. These two genera differ moreover in having a very 

 small ant-orbital opening, which is situated far forward ; in 

 the former there is a projecting fold of bone which protects 

 the anterior outlet of this opening. In most Arvicolidce, the 

 malar bone is broad and vertically compressed; it is im- 

 mensely developed in the beaver, and unlike other species of 

 the present group, runs up to join the lachrymal bone. 1 On 

 the other hand, in the two genera Spalax and Geomys, it is 

 small and very slender. 



The anterior root of the zygoma is in the form of a thin 

 plate, of considerable extent. This plate is oblique in its 

 position, and its lower edge is emarginated as in the rats. 

 The genus Spalax forms an exception, this plate being of 

 but small extent. 



The incisive foramina are tolerably large in Ondatra, Ar- 

 vicola and Lemmus, but small in the remaining genera ; they 

 are always situated partly in the inter-maxillary and partly 

 in the maxillary bones, excepting in Geomys and Castor, 

 where they are confined to the inter-maxillaries. 



The palate is moderately broad and but slightly contracted 

 between the anterior molars, in Arvicola, Ondatra and Lem- 

 mus ; in Spalax and Geomys it is narrow, and in the beaver 

 it is much contracted between the anterior pair of molars, 

 but expands posteriorly. The skull in Geomys (fig. 71) is 

 remarkable for the peculiar form of the posterior portion of 

 the palate. The two pterygoid bones converge and meet in 

 front, where they expand, and joining with the palatine bones 

 form a horizontal platform, which is situated between the 

 hinder pair of molars, and considerably below the plane of 

 the palate ; opening on to this platform are two large fora- 

 mina, which are the outlets of two horizontal canals : these 

 canals run under the palatine bones, and open in front of 

 them, and are then continued forwards on the palatine portion 

 of the superior maxillary bone, in the form of two deep grooves. 

 A similar structure may be seen, but in a less marked degree, 

 in the common water-rat, and some other Arvicola. 



1 The malar bone of the heaver differs also from other Arvicolidce, inas- 

 much as it enters into the composition of the glenoid cavity. 



