78 



100 |im and height of 90 |im. The vehger valves have a straight hinge margin, 

 bearing several minute, equal-sized teeth (Figure 54). The larva attains the 

 veliconcha stage at a shell length of 120-130 |j.m. The shell is equivalve, 

 ovate, and highly convex. The anterior margin is pointed and only slightly 

 raised. The umbones are high and well developed. The ratio of length to height 

 and thickness of the valves is 1.6:1.3:1.0. The larva is dark brown and due to 

 the great convexity of the valves is less transparent. Striation on the shell is 

 concentric and distinct. The eyes are small and almost imperceptible against 

 the dark background of the shell (Figure 54). The hinge line is arcuate due to 

 the development of large teeth, five-six, at each margin of the provinculum. 

 There are 10-12 central rectangular denticles in the hinge row. The length of 

 the hinge line does not exceed the width of the umbones, but is equal to it, 

 reaching 100-110 |J.m in developed larvae. The ligament is posterior but, 

 unlike the ligament in other members of the family Mytilidae, shifted much 

 closer to the middle of the hinge line. The shell in the veliconcha stage ranges 

 from 120-280 |im in length. 



Ecology 



The pelagic stage in M. senhousia continues for two-three weeks. Along 

 the southern coast of Sakhalin (Aniva Bay, Sea of Okhotsk) and in Vostok 

 Bay, larvae are found in the plankton for a fairly long time — from the end 

 of July to September inclusive, with a maximum water temperature of 18- 

 23°C. 



Some data on the developmental biology of Musculista are available in 

 works of Morton (1974) and Matveeva (1975). Kulikova (1978, 1979) has 

 described the morphology of the pelagic larvae and their population dynamics 

 in the plankton of Busse Lagoon (southem Sakhalin). 



CRESCENT-SHAPED XDVl^K, ADULA FALCATOIDES HABE 



(Lithophagidae) 



Veliconcha 



The shell of the veliconcha is longitudinally oval and its length consider- 

 ably greater than its height. The ventral margin is almost parallel to the 

 posterior, the anterior margin only slightly produced dorsally, and the posterior 

 margin slightly produced ventrally. The shell is massive and strong. Concentric 

 striation is distinct, uniformly arranged, and even visible in live larvae. The 

 umbones are distinct — broad, round, and moderately high. The hinge system 

 is similar to that in the family Mytilidae but the smaller teeth in the central 

 part of the provinculum are either not visible or very weakly developed. The 



