508 University of California Publications in Botany [Vol. 8 



Crouan seems to us to be more properly included in the Ectocarpaceae, 

 as a further development of such forms as Ectocarpus hemisphericus 

 and E. chantransioides and on the way toward simpler ^^giraceae, such 

 as Ave conceive may be represented by Microcoryne (cf. Oltmanns, loc. 

 clt., pp. 25, 26). Strepsithalia is unknown to us, but on account of its 

 gelatinous character may possibly be considered to be a slightly 

 developed, or even degenerate relative of Corynoplilaea. Corynoplilaea, 

 if it be distinguished from Leathesia, is more generally solid, with 

 longer assimilating filaments which are also less swollen at the tip ; 

 Kylin (1907, p. 83) is of the opinion that the three genera, 3licro- 

 coryne, Corynoplilaea, and Leathesia are to be retained as representing 

 a series in complexity, leading up to the most highly developed species 

 of Leathesia. As indicated above, Microcoryne, on account of its elon- 

 gated form and greater or less differentiation of layers seems to 

 approach JEgira and the resemblance in structure to Mgira has been 

 generally noticed and commented upon. Between Leathesia and Cory- 

 noplilaea there seems to be no satisfactory distinction either as between 

 solid and hollow, or between longer and shorter cortical filaments. We 

 have therefore merged the two genera under Leathesia. and designated 

 the family as Leathesiaceae. AVe can see no resemblances sufficiently 

 close to unite Petrospongium with Cylindrocarpus (considering C. 

 microscopiciis Crouan as the type). We have therefore kept them 

 distinct for reasons to be given later. 



The Leathesiaceae are carnose algae of flattened or globular form, 

 solid or hollow, arising from a primitive basal disk, with the inner cells 

 large and colorless, more or less agglutinated, the cortical cells in 

 short filaments with chromatophores, without exserted structures of 

 any kind except colorless hairs, with peripheral growth, and having 

 gametangia and zoosporangia on the same or on similar plants. 



Key to the Genera 



1. Zoosporangia more or less difform, usually attached laterally 



,.15. Petrospongium (p. 508) 



2. Zoosporangia pjTiform to ellipsoidal, attached at the base. 16. Leathesia (p. 510) 



15. Petrospongium Naeg. 



Thallus small, thin, flat, usually encrusting rocks, with or without 

 rhizoids, gelatinous, composed of 3 more or less distinct tissues, viz. ;■ 



(1) a basal layer, monostromatic ( ?), giving off rhizoids, whence arise 



(2) loosely compacted, di- or trichotomous filaments, decumbent below, 

 curving to erect above, consisting of elongated, cylindrical or swollen. 



