332 MARINE BIOTIC COMMUNITIES 



tesselatus Ives), the rose star {Crossastcr papposus [L.]), and the 

 basket star {Gorgonocephalus euclenis M. & T.). There are also char- 

 acteristic species of brachiopods, hydroids, pecten, and of crabs and 

 shrimps. The rat-tailed fish {Asterotheca alascona [Gilbert]) and 

 Gilbert's sculpin {Gilbertidia sigolutes [J, & S.]) are perhaps most 

 common among the several species here. The species listed on page 330 

 as characteristic of the whole major community are usually present in 

 abundance. 



Faciations and Relations. This association is characterized by a 

 Modiolus faciation, which covers mud bottoms in the deeper parts of 

 the continental shelf in which the hydroclimate is suitable for the 

 biome (Strongylocentrotus-Argobuccinura). The shells form a hard 

 bottom on which the other dominants may rest. Again some depres- 

 sions may have been filled with silt and shells which support the 

 dominants. A sere may be traced beginning with burrowers in the 

 mud of such depressions. These are later smothered out by IModiolus, 

 and this in turn eventually gives way to shells of dead animals which 

 are used as a resting place for the biome constituents. 



Bivalve-Worm Communities 



Two major communities of the Puget Sound area in the North Pa- 

 cific belong to this type; one is characterized by mollusks of the 

 genera Macoma and Paphia and the other by two other mollusks be- 

 longing to the genera Pandora and Yoldia, the former being in shal- 

 lower water than the latter. Both stand out in contrast to the bar- 

 nacle-gastropod communities because of the less showy and generally 

 smaller size of most of the constituents, as well as the striking dif- 

 ferences in life form and life habit. However, neither has been studied 

 sufficiently to be named with certainty. Communities of the same two 

 types occur in the partially enclosed waters of both the North Pa- 

 cific and North Atlantic. Petersen's Macoma community (JMacoma- 

 Mya biome) and the Macoma-Paphia community are of the same 

 type, and the known facts regarding the two supplement each other 

 (cf. Huntsman, 1918; Ford, 1923; Hunt, 1925; Stephen, 1931, 1933). 



