12 



EAST COAST MARINE SHELLS 



Fig. 5 

 Sinistral imivalve, Busycon peirversum 



The Shell . In classification the 

 shell, which actually is an organ of the 

 molluscan body, plays a very important part. 



Shells are EXTERNAL when contain- 

 ing all or a portion of the animal, INTER- 

 NAL when enclosed by the mantle. 



The calcareous portion of the shell 

 always has an organic base, first secreted 

 by the animal and then impregnated with 

 carbonate of lime which is obtained in the 

 food supply. It is well known that vege- 

 tables derive their elements from the min- 

 eral kingdom (air, water, and earth) while 

 the animals obtain their elements from the 

 vegetables. When there is a deficiency of 

 lime the shells are thin and delicate, 

 while on the other hand an abundance pro- 

 duces massive or abnormally thickened spec- 

 imens. 



The organic base is called CONCHYO- 

 LIN and maintains the utility of the shell 

 during the life of the owner. After death 

 of the occupant the shell soon becomes 

 brittle and unattractive in appearance. 

 When fresh the specific gravity of the con- 

 chyolin exceeds that of Carrara marble. 



The disposition of the shelly mat- 

 ter in thin layers, which extend over one 



another, diffract the light and cause the 

 iridescent effect; the thinner or more 

 transparent plates affording a be£.utiful 

 lustre. 



The texture of shells varies con- 

 siderably, some presenting a dull lustre 

 like marble; these are called PORCELLANOUS . 

 Others are pearly, fibrous, horny, or glass- 

 like. 



The NACREOUS shells, or pearly ones, 

 when polished supply the "mother-of-pearl." 

 This is the most perishable type and conse- 

 quently among fossils only fragments usual- 

 ly remain. 



Among the FIBRODS shells are layers 

 of prismatic cells. When very thick they 

 break up vertically into pieces. 



The oyster with its LAMINATED struc- 

 ture separates into horizontal plates or 

 pieces. 



The Epidermis , or periostracum is 

 an outer skin which frequently covers the 

 surface of the shell and acts as a protec- 

 tion against external destructive agents 

 such as acids in the water. It is extreme- 

 ly variable in form. 

 Often it is very thin, 

 thus revealing the color 

 pattern beneath, or it 

 may be thick and opaque. 

 Certain species possess 

 a silky covering while 

 others are hairy or with 

 the appearance of coarse 

 cloth. Trichotropis 

 borealis, shown in Fig. 

 6, has a yellowish epi- 

 dermis which rises like 

 a bristly fringe along 

 the keels of the shell. A bearded aspect 

 is assumed by other species. In Cypraea 

 the epidermis, when present, is covered by 

 an additional layer of shell. 



The epidermis soon fades and drops 

 off the shell after the death of the ani- 

 mal. 



Whorls of Univalves . A whorl is 

 one complete revolution of the shell around 

 the axis, the last one which completes the 

 shell being the BODY WHORL. The number of 

 whorls vary greatly in different species. 



Sculpture . This is a very impor- 

 tant character. A shell is STRIATE when 

 parallel lines are fine and close together. 



I 



Fig. 6 

 Bristly epidermis 



