12 CALIFORNIA SEA-SIIELLS. 



front of the umbones, is called the lunule. To 

 learn these various parts, compare the following 

 explanation. 



EXPLANATION OF PLATE I. 



Fig. 1, A Univalve ; Purpura crispata. a, 

 apex ; $p, spire ; ap y aperture ; o. Z, outer lip ; ca, 

 canal; u, umbilicus; c, columella; b. w, body 

 whorl ; 8, suture. 



Fig'. 2, A Bivalve, Tapes tenerrima. u, umbo ; 

 lu, lunule ; #, a, adductor muscular impressions ; 

 p y pallial line ; p. 8, pallial sinus ; I. t, lateral 

 tooth ; c, cardinal teeth ; I, ligament 



Mollnsks, like plants, have two names ; the 

 first is the generic, or the name of the genus ; 

 the second is the specific, or the name of the 

 species. For example, Mytilus Calif or nianus^ 

 the " California mussel." By a species is meant a 

 group of individuals which have such similarities 

 that they may be supposed to have descended 

 from a single pair. A genus is a group of sim- 

 ilar species. The abbreviation after a name des- 

 ignates the author who first gave that name to 

 the species. 



