298 rniversity of California Publications in Botany [Vol.8 



Tlie shells of Ilyanassa obsoleta were introduced some years ago 

 along with oysters from the Atlantic coast of North America, and 

 possibly the plant here described was introduced with the host. 



The comparison of this species with others will be found included 

 in the discussion under P. applanata. 



3. Pseudulvella applanata S. and G. 



Thallus thin, parenchymatous, spreading by marginal growth, 

 expanded to several mm. in diam., smooth and glossy, 45-55/x thick; 

 color grass green ; cells in fairly regular vertical rows nearly iso- 

 diametric, sharply angled, 6-7. 5ju, diam. ; chromatophore covering the 

 cell wall, provided with one pyrenoid; zoosporangia ( ?) slightly modi- 

 fied surface cells ; reproduction imknown. 



Growing on the shells of Littorina planaxis Nutt. Central Cali- 

 fornia. 



Setchell and Gardner, Phyc. Cont. I, 1920, p. 295. 



Littorina planaxis is very abundant in small tide pools and in 

 moist places along high-tide level from Sitka to San Diego. Pseudul- 

 vella applanata has been studied only on material found along the 

 coast of central California, but it is presumed to have a much wider 

 distribution. 



Its presence on the host is readily recognizable by its expanded, 

 grass green, glossy appearance. Microscopically it may readily be 

 distinguished from all other described species of the genus by its very 

 small, closely compact, parenchymatous cells, and its seeming absence 

 of radiating filaments composing the basal layer. It spreads over 

 the host by tangential and by radial divisions of the peripheral cells, 

 at least it can thus be stated when the plant is of considerable size. 

 It probably starts on the very young host, and doubtless many plants 

 early coalesce to form a confluent thallus. We have never been able 

 to ascertain the nature of the early developmental stages, although 

 even very small shells have been examined. 



Reproductive bodies have been seen to escape from the surface 

 cells. Wliether they are zoospores or gametes, the number of cilia they 

 posses, and their behavior after escaping are subjects for furth(;r 

 investigation. 



The three species of Pseudulvella treated of in this paper may 

 be arranged, so far as the basal layer is concerned, in the follow- 

 ing sequence : P. prostrata, with distinctly radiating basal filaments 



