PATELLID^. 419 



grounds, to associate them in one family, and in doing 

 so feel at the same time, that, without making any 

 scientific sacrifice, we are relieving the conchologist of what 

 seemed to him one of the most anomalous and unnatural 

 disunions of similar shells, which the naturalist, through 

 overvaluing the characters presented by the respiratory 

 organs in the Limpet tribe, had proposed. Between the 

 genera themselves, however, it is very difficult to dis- 

 tinguish by shell alone. 



The animals of the Patella tribe have distinct heads 

 furnished with two distinct tentacles, which are in some 

 genera provided with eyes towards their external bases, 

 in others are eyeless. They have a very large and power- 

 ful creeping disk, between the sides of which and the 

 mantle are seen, in some genera, the branchiee forming a 

 cordon of fine lamellae ; in others these organs are grouped 

 into a distinct plume, and lodged in a cervical cavity. 

 The sides of the foot are never ornamented with cirrhi, 

 but the margin of the mantle is sometimes cirrhated, 

 sometimes entire. The mouth is armed with a pair of 

 corneous jaws, between which we find the extremity of 

 a very long ribband- like tongue, bearing a powerful ar- 

 mature of denticles. The arrangement of the denticles 

 is constant in each genus. The shell is conical and 

 cup-shaped: it is entire, and its apex is usually turned 

 towards the head of the animal, in one genus from it. 

 On the inner surface of the shell are seen the muscular 

 impressions from which the position of the head of the 

 animal may be known when the soft parts are removed. 



