350 Annals of the Carnegie Museum. 



Another difference between the two habitats is to be noted in the 

 constant clearness and transparency of the waters of the lagoons, 

 whereas the lake is often distinctly turbid. 



To briefly sum up the differences between the environment afforded 

 by the lagoons and the lake ; the former (a') is warmer during the 

 growing season, but (/5) has a greater variation of temperature, (c) is 

 free from currents and mechanically violent waves, and (d) has a 

 greater amount of insolation below the surface. 



The structure of the Potamogeton formation is typically : 



Fades. — 



Potamogeton pectinatus. 



Prmcipal Species. — 



Potamogeton piisilliis, Vallisneria spiralis. 



Fruiting specimens of Potamogeton pectinatus appear very early in the 

 life* of the lagoon, and perhaps may represent simply a continuation 

 of a formation of the lake itself. Probably many of the plants of the 

 lagoon are derived directly from the lake by the separation of the 

 lagoon from it. Other plants were likely derived from disseminules 

 which were buried by wave-action in the sand forming the bottom of 

 the lagoon. The facies of the formation appears mainly in the deeper 

 part of the lagoon where the depth is three feet or more. The princi- 

 pal species, however, are quite abundant in the shallower water, even 

 where not over eight or nine inches deep. In Lagoon Aa the plants were 

 spreading rapidly by rhizomes, which were buried about an inch in the 

 sand, and which were sending up rosettes at intervals of a few inches. 

 The rosettes in the shallow water near the shore were probably buried 

 by indrifting sand before they attained maturity. 



The Popubis-Salix Formation. 



During periods of wet weather or of high water in the lake the water 

 in the lagoons quite frequently rises sufficiently to inundate the zone 

 of Cottonwood and willow seedlings which at this stage constitutes the 

 Popiilus-Salix formation. Around Lagoon Aa the ecesis of these two 

 species was accomplished during the summer following the segregation 

 of the lagoon from the lake. In places on the west side of the lagoon 

 the formation is about thirty feet wide and consists almost entirely of 

 seedlings in their fourth year in 1906 (see Plate XL). The indrifting 



