16 



EAST COAST MARINE SHELLS 



Fig. 20 

 Alate or winged shell, Area wag- 

 neriana, Florida Pliocene 



Relative Size of Valves 



Bivalves generally are EQDIVALVE, 

 the right and left valves corresponding in 

 form and size. INEQUIVALVE forms are occa- 

 sionally met with, for example PI. 60, Fig. 

 5a. 



The majority are INEQUILATERAL, 

 more or less unequal sided, with the umbo 

 toward one end and the anterior side usual- 

 ly the shortest. Fig. 19. When the umbo is 

 situated near the center of a symmetrical- 

 ly formed shell, with corresponding area of 

 anterior and posterior side, the shell is 

 called EQUILATERAL, PI. 54, Fig. 3. 



ALATE 

 or "winged" 

 shells are 

 formed by ex- 

 tension of the 

 dorsal borders 

 as in Pecten, 

 PI. 8, Fig. 3. 

 In some cases 

 these "wings" 

 or "ears" are 

 much longer or 

 larger, Fig. 20. 



The term CLOSE Is applied when the 

 shells fit accurately together at the mar- 

 gins and appear to seal the shell. When 

 open spaces are present, the fit not ac- 

 curate or complete, the shell is GAPING. 



The Umbones . One of the pair is 

 called the UMBO, Fig. 19f. This term is 

 used to designate the prominent part or 

 "apex" of each valve. It is formed around 

 the embryonic shell. As growth progresses 

 the umbones naturally become wider apart. 

 They frequently possess totally different 

 sculpture than the subsequent growth and 

 offer important and dependable characters 

 in the arrangement of species in natural 

 groups. 



The umbones usually point forward 

 but there are a few exceptions In which 

 they point backward (Donax and Semele among 

 others) . 



The Rostrum or Beak . This is the 

 produced posterior end of bivalve shells, 

 often accentuated by bluntly angular ridges 

 which follow to the umbones, PI. 60, Fig. 

 6b, 



The Lunule . The lunule is an oval, 



often heart-shaped, indented 

 space in front of, or anterior 

 to, the umbones. Fig. 21. In 

 separated valves this space is 

 called the ANTERIOR SINUS but 

 it is present in both valves. 

 Many species do not possess 

 the lunule. 



Fig. 21 The Hinge and Its 



Lunule, hearts Parts . The active bivalves 

 shaped area usually have the strongest 



hinges; the sluggish forms, or 

 those which are fixed during life, the weak- 

 ly formed or toothless ones. 



The hinge itself is on the margin 

 and composed of chitinous ligament and 

 teeth (denticles) which closely interlock. 

 There is great variation among the 

 HINGE TEETH. Juveniles show well-defined 

 characters and are better for study. Old 

 individuals often partially cover the teeth 

 with shell deposit and they are consequent- 

 ly ill defined. The CARDINAL TEETH are 

 placed Immediately below or between the um- 

 bones. Fig. 19h. On each side of the cen- 

 tral teeth, or cardinals, are the LATERAL 

 TEETH which often are well apart from the 

 others. The ANTERIOR-LATERAL, Fig. 19J, is 

 the one in front of the shell, while the 

 POSTERIOR-LATERAL, Fig. 19k is the one in 

 the rear. While the cardinals are some- 

 times very small, or absent altogether, it 

 is more frequent to find the lateral teeth 

 not present. In Area the usual teeth are 

 lacking and are replaced with a large num- 

 ber of regularly arranged small teeth. The 

 latter are usually feeble toward the umbo. 

 The hinge of Cardium is shown in Fig. 22. 



All sorts of com- 

 binations and develop- 

 ments occur among the 

 hinge teeth. Upon these 

 depend, to a large ex- 

 tent, the identifications 

 of many species. 



The Muscular Im - 

 Fig. 22 pressions . These indenta- 

 Hinge of Cardium, tions inside bivalves 

 exhibiting arched often indicate the condi- 



teeth tion and position of the 

 locomotive and respiratory functions of the 

 organism. The two most prominent are known 

 as the ADDUCTOR MUSCLES. The ANTERIOR AD- 

 DUCTOR is shown in Fig. 19jf, the POSTERIOR 

 ADDUCTOR in Fig, 19m. The line connecting 



