144 LLOYD'S NATURAL HISTORY. 



ascend rivers, and a few are found in land-locked lakes. Al- 

 though their movements on land are awkward and ungainly in 

 the extreme, the members of one family (the Eared Seals, of 

 which there are no British representatives) pass the whole of 

 the breeding season on shore, during which prolonged period 

 they undergo a complete fast. 



The true Seals, or Phocidm, which are the most specialised 

 representatives of the entire group, are characterised by the 

 circumstance that the hind-flippers, when on land, are ex- 

 .ended backwards parallel to the tail, and are thus incapable 

 of taking any share in those movements which, by courtesy, 

 may be termed walking ; this mode of progression being 

 effected mainly or entirely by a kind of jumping movement of 

 the "body. All these Seals are further characterised by the 

 total absence of any externally projecting ear; the passages to 

 the brain opening flat on the surface of the sides of the head. 

 Although the number of incisor teeth is subject to some 

 degree of variation, the pre-molars and molars collectively 

 always form five pairs in each jaw; these teeth generally 

 having three well-marked cusps arranged longitudinally, with 

 sometimes a smaller fourth cusp posteriorly, but being occa- 

 sionally simple. 



The British Seals, aHhough not numerous, include represen- 

 tatives of three distinct genera. 



GENUS HALICHffiRUS. 

 Halichmrus, Nilsson, Faun. Skandinav. vol. i. p. 377 (1820). 

 Three pairs of upper, and two of lower incisor teeth; cheek- 

 teeth {i.e., molars and pre-molars) mostly with single roots, 

 and simply conical without accessory cusps. First and fifth 

 toes of the hinder flippers not much longer than the three 

 middle ones ; and the webs on their feet not projecting be- 

 yond the extremities of the toes. 



