190 



ZOOLOGY 



/ 3 . Shell under 18 mm. ; promi- 

 nent external ligament, 

 2 . Shell not at all gaping 



bi. With not over 3 cardinal teeth in each valve. 

 t'i. Valves equal in size, curvature, and 



markings. 



di. Anterior muscular impression very 

 small, hinder large; shell elongated, 

 mussel-shaped ; umbo at or near 

 posterior end .... 



d. Hinge line straight, formed by ears 

 at sides of umbo, radiating ribs, 

 scalloped edge .... 



d$. Height and length nearly equal ; a 

 decided pallial sinus 



(Hi 



d. Height and length nearly equal ; no 

 distinct pallial sinus, at most a 

 mere undulation of pallial line at 

 posterior end. 



f\. With a plain lunule in front of 

 umbones; without radial 

 ribs. 

 fi. Over 20 mm. long, com- 



/ * O 7 



pressed , ligament external 

 /_. Shell somewhat quadrant 

 shaped; less than 8 mm. 

 long; surface with con- 

 centric ridges. 



/s. Shell less than 5 mm. long, 

 very much compressed ; 

 ends rounded . 



e 2 . With no lunule and with dis- 

 tinct radial ribs . 

 Co. Valves unsymmetrical. 



(?i. 2 muscular impressions, a distinct 



pallial sinus. (See t? 2 under !.) 

 d. 1 muscular impression ; height never 

 much less than length 



Tellinidce 



Mytilidce 



(Mussels) 



Pectinidce 



(Scallops) 



Veneridce 



inl-shelled Clams) 



As t art i < he 



Crassatellidce 



ErycinidcK 

 Oardiidce 



