Dr. W. Dybowski on Lubomirskla baicalensls. 31 



tbey orio-in.ite only from one side of the stem Csemipinnate') 

 (see ri. II. fig. 1 c). 

 b. Bush-liliB sponges. When numerous twigs grow forth in various 

 directions from a short and thick base, a bush-like form is pro- 

 duced (PI. II. fig. 1_ a). In such a bush we observe very 

 differently formed twigs. Most of them are furcate and grow 

 together in their lower part. The size of a bush is somefimcs 

 enormous. 



With regard to the occurrence and distribution of tlie 

 sponges in Lake Baikal, my brother sends me the following 

 information * : — • 



In the south-western portion of Lake Baikal f, that is in 

 the whole stretch from Listwiennischnaja and Possolsk on 

 the one hand, to Kultuk on the other, the sponges occur 

 wherever the necessary conditions are present \. 



As a rule the sponges occur wherever the bottom of the 

 lake is stony and where large blocks of rock or wood are 

 lying about on it ; further numerous sponge-stocks are found 

 in those places where the steep rocky shore forms terraces 

 projecting into the water in steps. On the other hand, if the 

 bottom be sandy, muchly^ or covered with small easily movable 

 stones, no sponges occur. At a depth of 100 metres there- 

 fore, where the bottom is always covered with fine mud, a 

 few small coating stocks occur only where large blocks of 

 stone or logs of wood project out of the mud. On the western 

 shore these conditions are abundantly realized, so that here 

 sponges are almost everywhere met with. Close to the 

 shore, and in only inconsiderable depths, turf-like or cushion- 

 like sponge-stocks exclusively occur ; globular ones are rave, 

 arborescent forms are never found. The sponges usually are 

 firmly attached to the surface of large pieces of wood or rock ; 

 they show, however, a special inclination for rotting wood, 

 so that in certain places almost every log of wood bears 

 sponges. Sometimes one finds large logs which are regularly 

 coated Avith a crust of sponges. 



Li considerable depths, as, for example, at a depth of 3 

 fathoms ( = 6*3 metres), often quite close to the steep rocky 

 shore, bush-like sponges exclusively occur. In fine still 

 water, when the surface of the lake is as smooth as a 



* In the absence of a monographic treatise on the Baikal sponges the 

 observer only distinguishes the arborescent from the turf-like or cushion- 

 like stocks. The separate species or varieties are not specially treated, 

 but only the conditions of the Baikal sponges in general. 



t See Dybowski, I. c. p. 7, fig. 1. 



X Extract from a letter dated " Irkutsk, 1878." 



