190 



ZOOLOGY 



/ 3 . Shell under 18 mm.; promi- 

 nent external ligament, 

 2 . Shell not at all gaping. 



b\. With not over 3 cardinal teeth in each valve. 

 ci. Valves equal in size, curvature, and 



markings. 



d\. Anterior muscular impression very 

 small, hinder large; shell elongated, 

 mussel-shaped ; umbo at or near 

 posterior end .... 



<? 2 . Hinge line straight, formed by ears 

 at sides of umbo, radiating ribs, 

 scalloped edge .... 



Tellinidas 



MytUidcB 



(Mussels) 



Pectinidce 



(Scallops) 



Co. 



(7 3 . Height and length nearly equal ; a 



decided pallial sinus . . . Veneridce 



(Hard-shelled Clams) 



d. Height and length nearly equal ; no 

 distinct pallial sinus, at most a 

 mere undulation of pallial line at 

 posterior end. 



e\. With a plain lunule in front of 

 umbones; without radial 

 ribs. 



/i. Over 20 mm. long, com- 

 pressed, ligament external Astartidce 

 / 2 . Shell somewhat quadrant 

 shaped ; less than 8 mm. 

 long; surface with con- 

 centric ridges. . . CrassateUidce 

 / 3 . Shell less than 5 mm. long, 

 very much compressed ; 

 ends rounded . . ' jp; Erycinidce 

 e^ With no lunule and with dis- 

 tinct radial ribs . . . Cardiidca 

 Valves unsymmetrical. 

 d\. 2 muscular impressions, a distinct 



pallial sinus. (See d 2 under a\.) 

 rf 2 . 1 muscular impression ; height never 

 much less than length 



