EAST COAST MARINE SHELLS 



17 



these two is the PALLIAL LINE, Fig. 19n, 

 while the notch in the same line is called 

 the PALLIAL SINUS, Fig. 19o. 



The Ligament . This is an uncalci- 

 fied part of the shell, often attached to 

 ridges along the dorsal margin, posterior 

 to the umbones and uniting the valves, Fig. 

 19c. It is dark brown and very prominent 

 in Tellina alternata. It is plainly shown 

 in Macrocallista, PI. 54, Fig. 8. When the 

 ligament is inside the shell it is called 

 the RESILIUM. The black resilium of Spisu- 

 la, a common clam, is a good illustration. 

 When fresh and wet the ligament is flexible 

 but when dry must be broken to open the 

 valves. A little glycerine applied to cab- 

 inet specimens will facilitate examination. 

 The FOSSETTE is a receptacle for the liga- 

 ment. 



Sculpture . The external sculpture 

 of bivalves is often smooth but it may be 

 otherwise. Growth lines indicate succes- 

 sive stages in the development of the shelL 

 CONCENTRIC RIBS run parallel with the mar- 

 gin of the shell. They are prominent in 

 Anatlna canaliculata, PI. 26, Fig. 3. CON- 

 CENTRIC LINES are much finer and closer to- 

 gether. FOLIACEOUS refers to a surface re- 

 sembling overlapping leaves of a plant. 

 RADIATING RIBS are those which emenate from 

 the umbones and cross the concentric sculp- 

 ture. 



The INTERNAL SURFACE is usually 

 white, polished, frequently iridescent and 

 also sometimes beautifully tinted. In some 

 cases there are ribs present which strength- 

 en the shell and are not visible outside. 



The Byssus , m'hen present, is used 

 for attachment to some solid object. It may 

 be of coarse texture or silky as in Pinna 

 carnea which is shown on PI. 5, Fig. 1. Al- 

 ways there is an opening in one of the 

 valves for its passage, as may be seen in 

 Pecten. 



Methods of Reproduction 



Among mollusks no cases of asexual 

 reproduction have been recorded. Usually 

 the sexes are separate although one large 

 subclass of the Gastropoda are hermaphro- 

 ditic. In the most highly organized mol- 

 lusca the sexes are distinct. The develop- 



ment of a sexual arm in the male Argonauta 

 is significant. 



In some of the simpler types the 

 spermatozoa are simply discharged into the 

 sea and are inhaled with the respiratory 

 currents by the opposite sex. 



The oyster is said to lay about 

 10,000,000 eggs. Certain other mollusks 

 deposit very few eggs, especially among the 

 land and fresh-water forms. Some species 

 normally hatch eggs within the body of the 

 parent. Others deposit the eggs upon the 

 shells of their own kind. 



The de- 

 velopment of 

 the trochophore 

 stage has al- 

 ready been ex- 

 plained (page 



1). 



The eggs 

 themselves are 

 individual 

 units, often ar- 

 ranged in clus- 

 ters within a 

 protective case 

 shaped like a 

 long narrow 

 tape or ribbon. 

 An example of this may be seen in Busycon 

 perversus (Fig. 23). The globular-shaped 

 eggs, arranged in clusters, of the apple 

 snail, Pomacea paludosa, are frequently 

 seen in May upon grasses just above the wa- 

 termark of Florida canals and lakes. Fig. 

 50a, page 128. 



The Radula . After passing through 

 the jaw the food of the gastropod mollusk 

 comes in contact with the adjacent radula. 

 The latter is coiled like a watch spring, 

 near the jaw, and is used to scratch, tear, 

 or bore rather than to bite. Only a small 

 portion is used at a time and food passing 

 over it is carded small. It is also util- 

 ized as a tool for boring into other shells 

 in order to extract their contents. Evi- 

 dences of this work are frequently found 

 in empty shells which have been neatly 

 drilled, see Fig. 22, page 16. 



The radula is often like a flat 

 ribbon of varying length and breadth, often 

 colored yellow or red in front. Upon the 

 upper surface are teeth of various sizes, 

 number and arrangement, usually in symmetri- 

 cal rows. In the center of the ribbon the 



Fig. 23 

 Egg cases of Busycon per- 

 versum 



