120 UNIVALVES. 



there is no trace of any volutions, present the 

 intermediate grade between them and the shells 

 entirely destitute of spires. 



The animal of the Patella has a large fleshy 

 proboscis, and two tentacula, with the eyes on a 

 small elevation at their base ; the mouth has 

 cartilaginous plates for the mastication of its 

 food. It has a ventral foot, fleshy, and furnished 

 with numerous muscular filaments, which uni- 

 ting on the upper part of the mantle, form a 

 strong muscle by which the body adheres to the 

 shell. By the action of this muscle, the shell 

 can be brought close to the surface to which the 

 foot adheres, or removed to a distance. By 

 means of this instrument, the animal, when 

 desirous of changing its situation, sometimes 

 effects a considerable leap. Its locomotive 

 powers, however, are but little exercised : it 

 appears to live habitually in the same spot, and 

 rarely to perform any other movement than that 

 of slightly elevating the shell, that the water it 

 breathes may reach the respiratory organs. 

 These shells are generally found attached to 

 rocks, sea-weed, and other marine substances, 

 and generally in such numerous clusters that the 

 stones appear quite studded with them ; they 

 adhere with considerable tenacity, and are not 

 easily displaced. They fix themselves thus se- 

 curely, by applying their fleshy foot and the edge 

 of their mantle to the substance on which they 

 take up their abode. They form an internal 

 vacuum by the contraction of their muscles, and 

 the pressure of the external air then keeps them 



