154 AXEL A. OLSSON 



but never spinous. Ligament is internal, seated in a narrow, deeply grooved, 

 median resilifer or fossette flanked by a pair of large crural teeth, fluted or 

 corrugated on their sides and in their respective sockets. 



The Plicatulas have sometimes been referred to the Spondylidae from 

 which they differ by their more irregular, less Pecten-Mke shape, lacking 

 ears or auricles; also by the absence of a cardinal area in one valve and 

 nonspinous sculpture. Much distorted specimens are easily mistaken for 

 small oysters. 



Genus PLICATTJLA Lamarck, 1801 



Type species by subsequent designation, Anton, 1839, Spondylus 

 plicatus Linne or by Gray, 1847, Plicatula gibbosa Lamarck. 



Shell generally small, irregular or distorted, externally resembling an 

 oyster but internally with the hinge provided with interlocking crural 

 teeth. The valves are subequal, irregular, cemented by the beak or umbone 

 of either valve. The external surface weakly or strongly folded 

 or plicated. The cardinal area is small, subtrigonal, a little larger in 

 the right valve but not showing a persistant slit or groove for the resilifer 

 extending towards the beak as in Spondylus. The hinge has two strong, 

 fluted crural teeth which fit into sockets, the teeth in the right valve 

 bordering the resilifer pit. The valve margins may be smooth or pustulate 

 and usually form a narrow or wide band which is the exposed portion of 

 the outer vitreous layer, white in some species, dark-colored in others. The 

 simple pallial hne lies a short distance within the ventral margin. 



All species of Plicatula are distorted due to the irregularities of the 

 surface to which they have been cemented; in forms which show only a 

 small attachment scar, the free surface of the valves is generally strongly 

 sculptured with sharp plications and folds; other shells living in more 

 exposed situations, the area of attachment is large and may cover the 

 whole surface of the lower valve. Although some species of Plicatulas are 

 common, their small shells are often mistaken for small oysters, and if 

 worn and encrusted, do not have a high appeal to the general collector. 

 At the present time, and until larger series of reasonably well-preserved 

 material becomes available, the standing of the few species of Plicatula 

 described from the Panamic region cannot be properly assessed. 



Plicatula penlelllata Carpenter 



Plicatula Penic'tllata Carpenter, 1856, Cat. Mazatlan Shells, Brit. Mus. p. 155 Bay 



of Fonseca, Cuming.— Sowerby, 1873, Conch. Icon., vol. 19, Plicatula, pi. 1, fig. 



3.— Hertlein and Strong, 1946, Zoologica, vol. 31, pt. 2, p. 63. 



Shell variable in size and shape from nearly circular, flattened forms 

 to irregularly humped, elongate-ovate types, the lower valve is usually 

 broadly attached over most of its surface. Upper valve may show a few, 

 irregular, ill-defined riblets around the margin or the whole surface may 

 be flat and plain or with an occasional short, hollow, spinelike elevation 

 rising above it. The outer or upper layer of the shell has a subcellular 

 structure giving to it a slightly subtranslucent luster, colored with a light 

 flush of reddish brown arising from a close sprinkling of small reddish 

 streaks, lines or spots. Interior mostly white, or with a mottling of darker 

 color showing through the porcellaneous inner layer, the teeth colored brown. 



