The Thomy Oysters 



ventricose, the upper beak quite overhanging its mate, is rather 

 more elaborate in ornamentation than the previous one, though 

 made on the same plan. The primary spines are longer, and 

 flung out at various angles. More and finer ridges separate the 

 six principal ones. The colour is rose purple. This species has 

 the longest spines of all. Diameter, 3 inches. 



Habitat. — Pacific Islands. 



The Painted Spondylus (S. pidorum, Chemn.) has its six 

 main ribs very regularly set with tongue-shaped spines. Inter- 

 vening ridges have similar spines or sharp pointed prickles. Some 

 are coloured with shades of rose. More commonly they are rose 

 and orange, or a rich salmon yellow. Some that begin life with 

 a coloured shell abandon the scheme and produce a white border. 

 No species is more variable in colouring than this. It is fastened 

 to coral rocks at a depth of fifteen to eighteen fathoms. Length, 

 3 to 5 inches. Lower California, Mexico, Colombia. 



THE PLAITED SHELLS 

 Genus PLICATULA, Lam. 



Shell irregular, oblique, attached by umbo of right valve; 

 valves usually plaited. Few living species. 



The Plaited Shell (P. ramosa, Lam.) is a little flattened 

 clumsy bivalve, with rusty hair lines on the plaited surface. 

 The hinge locks by two teeth on the left (upper) valve which are 

 inserted between the two set wide apart on the right. These 

 sedentary mollusks appear to accumulate limy deposits on the 

 shells as age advances, and serpulae and barnacles become at- 

 tached. West Indian specimens are largest. Length, i inch. 



Habitat. — North Carolina southward. 



P. Mantelli, Lea., found on our Gulf coast as fragile flat 

 valves, indistinctly eared at the hinge, faintly ridged and delicately 

 marked with brown, are a trifle smaller than P. ramosa. 



THE FOOT SHELL 

 Genus PEDUM, Brug. 



Shell wedge-shaped, thin, nearly smooth, flattened, slightly 

 curved ; right valve convex and deeply notched for passage of 

 the byssus. A single species is known. 



406 



