PANAMIC-PACIFIC PELECYPODA 149 



front of middle. The valve is slightly convex, raised around the margin, 

 its surface finely pitted, white, and usually showing a slight iridescence. 

 Range — Forrester Island, Alaska, to the Gulf of California. 



Family ISOGNOMONIDAE 



The shell is monomyarian or provided with a single adductor scar in 

 each valve, usually aviculoid in shape, with or without a posterior wing, 

 and attached by byssal threads passing through a sinal gap in the anterior 

 margin below the beaks, the margins of the byssal sinus often much 

 thickened. The shells may be small or of large size, thin or heavy, the 

 left valve being generally somewhat more convex. The beaks are small, point- 

 ed, and placed at the extreme anterior end and pointed forward. Hinge line 

 straight, edentulous, and bordered externally by a cardinal area which may 

 be low or high, generally thickened, sometimes excessively so, and traversed 

 by a series of parallel, vertical pits or grooves to which the main part of 

 the ligament is attached. Inner layer of shell nacreous, often brilliant, the 

 outer layer prismatic, lamellar and often forming a dull or horn-colored 

 zone extended widely around the margins of the valves. 



One single genus in Panamic waters. 



Genus ISOGNOMON Solander, 1786 



{Pedalion Solander, 1786 MS.; Melina Retzius, 1788; Perna Bruguiere, 

 1789 and Lamarck, 1799 [not of Retzius, 1788] ). 



Type species by tautonymy, Ostrea isognomon Linne. 



With the characters of the family. 

 iHOgnomon chemnitziana (d'Orbigny) Plate 18, figures 2, 2a 



Perna chemnitziana d'Orbigny, 184S, in Sagra, Hist. I'lle Cuba, Mollusques, vol. 2, p. 346. 

 Perna bicolor C. B. Adams, 1845, Proc. Boston Soc. Nat. Hist, vol. 2, p. 9. — Clench and 



Turner, 1950, Occasional Papers on Mollusks, Mus. Comp. Zool., vol. 1, No. 



15, p. 260, pi. 48, figs. 6, 7. 

 Perna quadrangularis Reeve, 1858, Conch. Icon., vol. 11, Perna, pi. 2, fig. 6. Hab. — ? 

 Pedalion chemnitzianum (d'Orbigny), Hertlein and Strong, 1943, Zoologica, vol. 28, pt. 



3, pp. 166, 167, pi. 1, fig. 8. 



Shell usually small, irregular, seldom over 50 mm. in height, subsolid 

 over the middle of the disk, fragile, and thin in the ventral portion. The 

 outer surface is usually coarsely lamellose, formed by the overlapping of 

 several wide sheets, the final one projecting as a wide rim or tongue 

 beyond the edge of the pearly layer within. 



Common as a nestler, living in crevices and in abandoned burrows. 

 P. quadrangularis of Reeve, described without knowledge of locality, is 

 probably equivalent. Also common on the Atlantic side of the isthmus. 



Range — Mexico to Chile. Atlantic. Canal Zone: Venado Beach. 

 Ecuador: Esmeraldas; Manglaralto; Santa Elena. 



Superfamily PECTDTACEA 

 Family SPONDYLIDAE 



Shell monomyarian, with convex, unequal, pectiniform or ostreiform 

 valves, attached by the umbonal section of the right valve which is always 

 larger. The single adductor scar is placed a little behind the middle. Hinge 

 line straight with a pair of large, curved or hook-shaped crural teeth and 

 their sockets in each valve. The cardinal area is high and triangular in 

 the right valve, absent in the left. The ligament is almost wholly internal, 

 lodged in a deep pit between the crural teeth. Sculpture formed by strong 

 radial ribs, usually spinous. Warm water. 



