NATURAL HISTORY SOCIETY. 79 i 



i 



ligament oblique, aemi-internal ; epidermis not thick..., /<?. Lucinido'. ! 



-M. Shell somewhat quadrant sliaped; small, less than 8 long; surface i 



with concentric ridges 10. Crassatellidce . \ 



M. Shell not over ^5 long, very mut'h compressed; ends rounded I 



U. Ery cin idee . J 



K. Without toothed hinge or proper ligament, often with accessory valves ; j 



no distinct epidermis. (JT) \ 



N. With proper hinge, often distinctly toothed and with either external liga- | 



ment or internal cartilage, sometimes both. (O.) j 



O. Hinge with many small cardinal teeth in each valve 7. LedidcE. \ 



O. Hinge of not over 4 cardinal teeth in each valve. (P.) ' 



P. Elongated equivalve shells two or more times as long as high, gaping ' 



more or less at both ends. (S.) ; 



P. Equivalve shells not fully twice as long as high. (R.) I 



P. Inequivalve shells ; length and height about equal 21. Semelidce. \ 



P. Inequivalve shells ; decidedly longer but not twice as long as high. (Q.) 



Q. Over 25 long, thin; right valve much more convex than the left; \ 



umbones about central ; both external ligament and internal cartilage. \ 



25. Anatinidce. \ 



Q. Over 25 long ; right valve nearly flat ; whole shell so compressed that • 



the width is not over one-sixth the length 27. Pandoridce. \ 



Q. Over 50 long; slightly inequivalve; no external ligament; an in- ] 



ternal cartilage placed between a spoon-shaped process, projecting out '. 



of the left valve into a concavity under tlie umbo of the right valve. ; 



30. Myidw. ' 



Q. Not over 12 long, thick, gaping in front; umbones about central { 



29. Corhulidw. \ 



Q. Covered with radiating wrinkles, thin, fragile, pearly translucent; left j 



valve slightly larger 26. Lyonsiida\ \ 



Q. Other small shells not over 18 long 20. Telliiiidw. \ 



R. Shell somewhat triangular-shaped, with an internal cartilage between | 



deep triangular pits, similar in ench valve 24. Maetridw. 



R. Shell thin, rounded in front, narrowed and gaping behind ; cartilage pit 



shallow; external ligament short 21. Semelida\ 



i 

 R. Shell with a large external ligament ; pallial line a series of elongated ' 



pits 31. Saxicavidce. '\ 



S. Shells elongated 3-6 times as long as high ; epidermis polished ; i 



umbones not over one fourth from the anterior end ; ligament external, j 



23. Solenida\ \ 



