40 Marine Shells of the Western Coast of Florida 



Pinna the valves are medially sulcate; valves of Atrina lack such 

 sulcation. 



Genus ATRINA Gray, 1842 

 Atrina22 rlgida (Solander) PI. 3, fig. 15 



Length, 250; width, 130 mm. Shell with characters of the family; 

 olive-brown; outer surface bears about 15 elevated ribs, with inter- 

 calations of smaller ribs posteriorly; ribs not well developed toward 

 apex and ventral border. Each rib bears a series of sharply elevated, 

 tubular or semitubular spines; the spines — like the ribs — are lost on 

 the umbonal and ventral portions of the shell; a fine, long, silky 

 byssus. 



Atrina serrata-^ (Sowerby) PI. 3, fig. 16 



Length, 260; width, 140 mm. Shell with characters of the family. 

 A Httle larger than A. rigida with posterior margin less rounded. 

 Surface sculpture of closely placed ribs bearing deeply curved, semi- 

 erect scales which are much reduced over ventral and umbonal areas. 

 Both species of Atrina are common on the beaches after blows, 

 found from low tide mark to six fathoms. 



Family PTERIIDAE 



The modest exterior of the shells of the Pteriidae beHes the 

 family's claim to high d'Stinction, for one of its genera includes the 

 great pearl oyster of the South Seas and the Pinctada which responds 

 to the exquisite skill of Japanese pearl culture. The secretion of nacre 

 is a function common to all the members of this group. An occasional 

 free pearl, but oftener a baroque, is found in the native species of 

 Pinctada. 



The Pteriidae are mollusks of warm seas and moderate depths. 

 Their shells are variable in shape and size, inequivalve; inequilateral 

 — often winged with a byssal notch. 



2- Lat., atrium, an opening; rlgere, to be stiff. 

 '^'^ Lat., serra, a saw. 



