EAST COAST MARINE SHELLS 



13 



It is SPIRALLY STRIATE 





when the stronger 

 lines revolve with 

 the whorls and 

 AXIALLY STRIATE 

 when they cross 

 the whorls aore or 

 less parallel with 

 the lines of 

 growth. A CANCEL- 



• LATED surface is 

 shown in Fig. 7. 

 Often when all 

 other characters 

 fail the sculpture 

 of the nucleus 

 will determine the 

 position of a 



At the base of the 



Fig. 7 

 Csmcellated surface 



given species. 



The Omb'ilicus . 

 shell, corresponding to the navel, there is 

 often present a cavity 

 known as the umbilicus. It 

 may be WIDE as in Fig. 8, 

 CRENULATED as in Architec- 

 tonica, NARROW as in Lit- 

 torina, or DEEP as in Na- 

 tica. 



Occasionally the 

 Fig. 8 previous whorls are visi- 

 .\n umbilicus, ble; the shell is then 

 in center called UMBILICATE. If the 

 opening is small it is 

 PERFORATE, when a mere fissure RIMATE but 

 when columella is solid and no opening ap- 

 parent it is IMPERFORATE. 



The Columella . This central pillar 

 is that portion around which the spiral 



shell revolves; plaits or 

 folds are shown in Fig. 9. 

 The walls adjacent to the 

 columella are sometimes 

 broken down by absorption of 

 internal partitions thus af- 

 fording more space in a sin- 

 gle cavity. When the colu- 

 mella is TRUNCATE or notched 

 at its base, it usually in- 

 dicates carnivorous propen- 

 sities. 



Fig. 9 

 Plaits on 

 columella 



The Nucleus is that portion of the 

 shell which emerged from the egg or ovum. 

 It may consist of one or several whorls and 

 usually can be recognized on account of 



Fig. 10 

 Bulbous nucleus 



the different charac- 

 ter of its surface. In 

 many species it is 

 glassy. The distinc- 

 tive sculpture has al- 

 ready been referred to. 

 An example of a bulbous 

 nucleus in Voluta 

 mamilla, from Austra- 

 lia, is shown in Fig. 

 10. See also descrip- 

 tions of Volutidae, 

 p. 127. 



The Apex . The tip or beginning of 

 the shell which is the top of the univalve 

 illustrations shown in this volume. Fig. 

 11a. Only the French figure shells with 

 the apex pointed downward. 



11a. apex 



lib. suture — 



lip. periphery- - 

 lie varlx — 



lid. umbilicus 



llf. columella-" 

 llg. anterior canal 



•y llh. spire 



_ llj . posterior 

 canal (if present) 

 Ilk. outer lip 



^ 11m. mouth or 

 apterture 



Fig. 11 

 Illustrating the technical terms applied to the 

 various parts of a univalve shell 



The Spire. This embraces the en- 

 tire univalve shell with the exception of 

 the final whorl. Fig, llh. It may be DIS- 

 COIDAL, resembling a disk, as in Skenea, 

 elongated as in Terebra, or one of almost 

 endless intermediate stages. 



Base of Shell . The opposite ex- 

 tremity to apex; also called anterior end 

 of univalves or most recently built portion. 



Periphery is that portion of each 

 whorl which is most outwardly developed and 

 usually midway between suture and base. 

 Fig. lip. It may be more or less rounded 



