14 



EAST COAST MARINE SHELLS 



Fig. 12 



Callioatoma, 



showing carina 



or keeled. When very 

 sharp the edge is known 

 as the CARINA, Fig. 12 

 in the accompanying fig- 

 ure of Calliostoma har- 

 risi, a fossil species 

 from the southern states. 



Several of the 

 Astraeas are strongly 

 carinated species. 



Suture is that portion where the 

 whorls Join; see Fig. lib. It is largely 

 influenced by the convex or placate charac- 

 ters of the whorls. It may be CANALICULATE 

 or CHANNELLED when a broad and deep chan- 

 nel follows close to the junction of the 



whorls. This is well 

 illustrated in Busycon 

 canaliculatum, Fig. 13. 

 It is CRENDLATED when 

 the suture is inter- 

 rupted by indentations 

 which break the con- 

 tinuity. An example 

 of this is Pyramidel- 

 la crenulata, from 

 Florida. 



Fig. 13 

 Channelled suture in 

 Busycon canaliculata 



The Aperture is the last portion 

 formed and through which the animal emerges. 

 It may be ROUND, NARROW, or another shape. 

 Fig. 11m. Sometimes it is greatly con- 

 tracted with folds or teeth which it would 

 seem almost impossible for the animal to 

 pass without injury (see Pedipes mirabilis, 

 PI. 55, Fig. 8). The OUTER LIP is shown 

 in Fig. Ilk, the round peristome in PI. 32, 

 Fig. 3. 



In describing the component parts 

 of the aperture the length of the same is 

 considered parallel with the length of the 

 shell, the width transversely to this. The 

 various terms used in connection with the 

 aperture will be found explained in the 

 glossary. 



Varices . In certain families there 

 is a tendency for the animal to indicate 

 rest periods in shell building by periodic 

 thickenings of the lip. At each of these 

 stages the shell in consequence assumes a 

 mature aspect. When full growth is at- 

 tained these early thickenings of the lip 

 are often still apparent. There may be one 

 or more of these. One is known as a VARIX, 

 Fig. lie, but this term is never applied to 



the final or most recent lip. Several are 

 known as VARICES. Examples: Epitonium and 

 Gyrineum. In certain cases the varix may 

 assume the form of a hump. 



Position When Active . The Gastro- 

 pod mollusk when crawling, foot downward, 

 usually carries the shell in such a manner 

 that the apex points backward. The opercu- 

 lum, when present, is pushed to one side. 



Canals . When present in the shell 

 these may be observed adjacent to the aper- 

 ture. Holding a shell with the spire up- 

 ward the POSTERIOR CANAL is the upper one, 

 Fig. 11 j; the ANTERIOR CANAL the lower. 

 Fig. 11m. When the animal is moving for- 

 ward the former is in the rear, the latter 

 in front and closest to the head of the 

 mollusk. 



The Operculum . The door which 

 closes the aperture of many spiral shells 

 is attached to the animal. When a mollusk 

 possesses this appendage it is OPERCULATE. 

 When absent it is INOPERCULATE. The ma- 

 terial used in its construction is usually 

 horny but in some species it is shelly or 

 calcareous. 



The operculum is formed in the em- 

 bryo, within the egg, as is the first or 

 several whorls of the shell. The point 

 from which growth starts in the operculum 

 is called the NUCLEUS. While many of these 

 doors fit with accuracy others only partial- 

 ly block the entrance. Conus, having de- 

 veloped a poisonous bite, is less dependent 

 upon an operculum for protection against 

 intruders and in consequence the door has, 

 in certain species, degenerated greatly in 

 size. 



The various forms of the operculum 

 may be expressed in these terms: 



CONCENTRIC— Whtn it increases equally all 



around the nucleus. Fig. 16. 

 ARTICULATED — When projections correspond to 

 teeth in the shell, as in 

 Nerita, Fig. 14. 

 CLAW-SHAPED or UNGUICULATE— As in Strombus, 



Fig. 18. 

 PAUCISPIRAL— Few whorled as in Littorina. 

 EXCENTRIC — When nucleus is at edge and de- 

 velopment one-sided; Fig. 17. 

 SPIRAL — When growth is only on one side and 

 revolutions are made with growth, 

 Fig. 15. 



