172 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. VII. 



attached to rocks, pieces of dead timber, sticks, and to the branches of 

 trees which touch the water. The non-filamentous forms, or the blue- 

 green slimes, are very abundant, and with many of these are associated 

 filamentous forms. Some species grow in considerable quantities in 

 very warm water near the warm springs. The most conspicuous and 

 by far the most abundant of these blue-green forms is a species of 

 Clathrocystis. It is exceedingly abundant near the surface in the upper 

 half of the lake, or rather that portion above the railroad, but is not 

 noticeable to the naked eye on that portion of the lake below the 

 railroad. Associated with this form, in colonies, are several species 

 of Anacystis and Anab&na. Belonging to these genera are the char- 

 acteristic " Wasserbliite " plants, but these nowhere on the lake or 

 near its margins formed a scum on the surface, and so, in the strict 

 sense, "Wasserbliite" cannot be said to exist on the lake in January 

 and February, especially so on its lower half; although, in a broader 

 use of the term, the presence of Anabcena and Clathrocystis in such 

 abundance indicated that it did exist to a limited extent, at least, on 

 the upper half of the lake. " Wasserbliite " is considered by some 

 writers to be injurious to fishes, especially so in small stagnant ponds. 

 While this may be true in small ponds, it certainly is not in such large 

 bodies of water as this lake, where the surface is frequently agitated. 

 These forms do form a portion of the food supply of the fishes in 

 this lake, and no doubt furnish the important food supply of Ento- 

 mostraca. 



The fact that the water in Lake Amatitlan has about the same tem- 

 perature during the entire year would indicate that Plankton was 

 quite uniform in quantity at all times; a condition which is not true, 

 however, of our northern lakes. 



In Lake Amatitlan the Phytoplankton is much more abundant in 

 January and February than the Zooplankton. In Lake Atitlan the 

 reverse is true. Lake Atitlan, the larger lake, is about 1,000 feet 

 higher. Its water will average from one to two degrees cooler than 

 Lake Amatitlan. It has no outlet, and its drainage area is very small. 

 That portion of Lake Amatitlan which is the least disturbed by the 

 inflow and the outflow of water contains by far the greatest abundance 

 of Phytoplankton. 



The abundance of the blue-green algae previously mentioned on 

 onte portion of lake, and not on the other, is not well understood. 

 The only reason I am able to suggest why they are so much more 

 abundant in the upper part of the lake than in the lower is that the 

 drainage into this portion of the lake is small compared to that 

 received by the lower portion. The upper part of the lake becomes 



