152 American Seashelh 



The horn shells are intertidal mud-lovers. The shells are elongate and 

 with lo to 15 convex whorls. Axial ribs are more prominent on the early 

 whorls. Outer lip flares. Operculum horny, thin, paucispiral and with its 

 nucleus at the center. 



Subgenus Cerithideopsis Thiele 1929 

 Cerithidea costata da Costa Costate Horn Shell 



Plate 19U 



West coast of Florida and the West Indies. 



/4 inch in length, translucent, pale yellowish brown. With 9 to 1 2 very 

 convex whorls. Axial, curved ribs are round and distinct on the early whorls, 

 fading out on the last two whorls. No old varices present. A common 

 shallow-water, mud-loving species. 



The subspecies C. costata turrita Stearns, the Turret Horn Shell from 

 the Tampa-Sanibel region, has 15 to 20 (instead of 25 to 30) axial ribs on the 

 next to the last whorl. 



Cerithidea pliculosa Menke Plicate Horn Shell 



Plate ipt 



Texas, Louisiana and the West Indies. Not Florida. 



I inch in length, brownish black in color. 11 to 13 slightly convex 

 whorls. Several yellowish, former varices are present. Numerous spiral 

 threads make the axial ribs slightly nodulose. Locally common. It may yet 

 turn up in northwest Florida. 



Cerithidea scalari^orvns Say Ladder Horn Shell 



Plate 19X 



South Carolina to south half of Florida and the West Indies. 



% to I % inches in length. Pale russet-brown to slightly violaceous, usu- 

 ally with many conspicuous, dirty-white, spiral bands. 10 to 13 moderately 

 convex whorls. Many coarse, axial ribs present which stop abruptly below 

 the periphery of the whorl at a sharply marked, rounded spiral ridge. Base 

 of shell with 6 to 8 spiral ridges. No former varices. Common on mud flats. 



Cerithidea hegeivischi cali^ornica Haldeman California Horn Shell 



Bolinas Bay, California, to Lower California. 



I to I /4 inches in length, resembling our photo of C. pliculosa from the 

 Atlantic (pi. i9t). Whorls 11, spirally and weakly threaded, and axially 

 strongly ribbed (12 to 18 ribs per whorl). Dark-brown in color with i or 



