422 '" AXEL A. OLSSON 



Length 50 mm.; of dorsal ridge, 4.5 mm.; of ligament 6.9 mm.; height 

 of shell, 6.0 mm.; convexity, 1.6 mm.; chord of arcuation of dorsal margin, 

 approximately 0.5 mm. South Passage, Pearl Islands. AMNH. 73421. 

 (Hertlein and Strong, 1955.) 



According to Hertlein and Strong, this species is characterized by 

 the slight arcuation of its valves, a feature that is constant in all the speci- 

 mens examined and which serves to distinguish it easily from E. calijornicus 

 and other similar forms. 



Range — Panama. Panama: Pearl Islands. 

 Superfamily MTACACEA 

 Family MTACIDAE 



Burrowers in sand or mud, sometimes nestlers, usually with elongated 

 or ovate, subequal valves but often irregular or distorted in nestling forms, 

 the surface white, earthy or chalky, often with coarse concentric growth 

 sculpture and a covering periostracum. Ligament mostly internal; in the 

 left valve, the resilium is attached to a large projecting arm or chondro- 

 phoral plate, the resilifer being a rounded or sagittate scar bordered above 

 and behind by a small ridge (nymph); this chondrophoral arm fits into a 

 deep pit in the beak of the right valve, the scar of the resilifer is placed in 

 the roof of the umbone within, while above it on the posterior margin of 

 the valve, there is a smaller scar to which the external portion of the liga- 

 ment (tensilium) is attached. Hinge margin with or without teeth. In- 

 ternal margins of valve smooth. Pallial sinus present or wanting. 



Mya arenaria Linn6 Plate 77, figure 1 



Mya arenaria is not a member of the Panamic-Pacific fauna but an 

 enlarged figure of its hinge armature is introduced here to illustrate the 

 general pattern of ligament attachment characteristic of the Myacacea, 

 In the Myacidae and the Corbulidae, the ligament, combining both the 

 tensilium and the resilium, is attached to a plate or armlike process found 

 only in the left valve. This large plate carries on its upper surface two deep 

 scars, separated by a ridge, for the attachment of the two parts of the 

 ligament; the larger anterior scar represents the true chondrophore and 

 to which the resilium was attached; the smaller narrower scar on the pos- 

 terior side that of the tensilium. This plate extends like a large tooth under 

 the beak of the opposite or right valve where a similar divided scar is found 

 in the roof of the umbonal cavity directly under the beak, and to which 

 the two bands of the ligament, side by side, were attached. The ligament 

 of the Corbulidae follows the same general pattern but with variation in 

 detail as to genus and species. 



General key to the genera of Myacidae in this region 



I. Shell of medium size (length about 30 mm.), subovate, subequivalve. 

 No definite sinus. Burrowers in sand and mud. 



Genus Cryptomya 



II. Shell smaller, usually less than 10 mm., nestlers in crevices and aban- 

 doned burrows. Valves usually distorted. The pallial sinus is well 

 developed. 



1. Valves lediform, the anterior side longer, convex, the surface marked 

 with uniform, concentric sculpture. The chondrophoral plate is rela- 

 tively small, usually pointed behind. 



Austroplatyodon as a subgenus of Platyodon 



