. East coast marine shells : descriptions of shore mollusks together with many living below tide mark, from Maine to Texas inclusive, especially Florida, with more than one thousand drawings and photographs. Mollusks -- Atlantic Coast; Mollusks -- Florida. 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
. East coast marine shells : descriptions of shore mollusks together with many living below tide mark, from Maine to Texas inclusive, especially Florida, with more than one thousand drawings and photographs. Mollusks -- Atlantic Coast; Mollusks -- Florida. 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,. of
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