MOLLUSC A. 



275 



ORDER I. DIOTOCARDIA. 



In these forms the body retains its bilateral symmetry to 

 a considerable degree, and externally may appear perfectly 

 symmetrical. The name implies the 

 existence of two auricles to the heart. 

 In the limpets the shell is a flattened 

 cone; in the abalones it is somewhat ear- 

 shaped and very weakly spiral, but in 

 the top shells it is strongly spiral. The 

 abalones alone have any economic 

 value. Their shells, remarkable for 

 having a series of holes in them, are 

 composed of a greenish mother-of-pearl, 

 which is extensively used in inlaid 



Work. FIG. 125. Limpet (Ac- 



meed testudinalis). 



ORDER II. MONOTOCARDIA. Gould Bhme y' s 



Here belong the great majority of marine snails, all of 



which agree in having but a single gill and a single auricle 

 to the heart. Few of them have any 

 economic interest aside from those which 

 feed upon oysters and other valuable 

 shellfish. These injurious forms com- 

 monly known as "drills" are able to 

 bore holes through the shells of oysters, 

 etc., by means of their lingual ribbons. 

 Many, however, are great favorites with 

 collectors, among them the strombs, 

 cones, cowries, and olives. Some of the 



FIG. 126. stromb cones are noticeable from the fact that 



(Strombus pugilis). 



After Woodward, they have a poison-gland connected with 

 the lingual ribbon. Some species formerly grouped as a 



