PANAMIC-PACIFIC PELECYPODA 143 



Order ISODONTIDA 

 Superfamily FTERIACEA 

 Family PINNtDAE 



The adult shell has a narrow, pointed anterior end, while the posterior 

 side may be elongated, expanded or fan-shaped, its margin normally open. 

 The shell is composed of two layers of different composition; an outer, pris- 

 matic, more or less flexible, perishable layer, and an inner, nacreous and 

 more durable layer, not formed along the growing ventral margin, conse- 

 quently, the ventral part of the valves is more or less flexible, and the 

 margins of the valves, normally gaping, can be closed tightly by the pull 

 of the mantle and of the posterior adductor muscle. These clams are 

 sedentary, generally found buried in mud, sand or gravel, the anterior end 

 downward and anchored in a vertical position by the byssus, the posterior 

 margin of the valves level with the surface or projecting slightly above it. 

 Hinge is long, narrowly linear, extending along a straight, dorsal margin, the 

 ligament produced to the end of the inner layer only. Some species of Pinna 

 such as rugosa attain a length of nearly two feet. The young Pinna is said 

 to have a normal, equilateral shell. 



Two genera separated by the presence or absence of a longitudinal keel. 



Key to the genera of Pinnidae 



I. Shell narrowly triangular, the dorsal and ventral margins nearly straight, 

 the anterior portion carrying a longitudinal carina formed in the outer 

 layer and dividing the inner layer into two lobes. 



Genus Pinna 



II. Shell fan-shaped, the posterior portion expanded. Valves not medially 

 sulcated. 



Genus Atrina 

 Genus PONA Linn6, 1758 



Type species by absolute tautonomy, Concha -pinna Hasselquist (=:Pin- 

 na muricata Linne; or by subsequent designation, Children, 1823, or by 

 Gray, 1847, Pinna rudis Linne). 



With the general chaiacters of the family. Surface of valves with a 

 longitudinal ridge due to a deep sulcus dividing the inner or nacreous 

 layer. 



The selection of the type species of this Linnean genus is controversial; 

 for the most recent opinion and a discussion of the subject, the reader is 

 referred to Turner and Rosewater, 1958, "The family Pinnidae in the 

 western Atlantic" (Johnsonia, vol. 3, No. 38, pp. 302, 303). 



Pinna rugosa Sowerby Plate 18, figure 1 



Pinna rugosa Sowerby, 1835, Proc. Zool. Soc. London, p. 84. Hab. in Sinum Panamensis 

 (Isle of Rey). — Reeve, 1858, Conch. Icon., vol. 11, A<vicula, pi. 26, fig. 50. — 

 Hertlein and Strong, 1943, Zoologica, vol. 28, pt. 3, No, 19, p. 165. 



Elongate, narrowly wedge-shaped, expanding posteriorly in large speci- 

 mens, the hinge line straight, held by the ligament along its whole length, 

 the resilium comprising about three-fourths of the area. Color brown, amber 

 shading towards black, often greenish near the tip, the median carina 

 showing as a dark line. Young shells are thin, subtranslucent, thickening 

 in the adult, sometimes excessively so. Sculpture formed by 6 to 10 rows 

 of widely spaced, tubular spines, largest along the middle and towards the 

 posterior end, covering the whole width, anteriorly the spines are absent, and 



