92 M. FOSLIE. [1905 



this is perhaps a mere accident, as the alga in other respects re- 

 sembles the form last-mentioned. Cp. Norw. Lithoth. p. 138, t. 22, 

 fig. 8. 1 ) — The Lithoth. durum recorded from Behring's Sea still 

 seems to me to have to be referred at any rate partly to the spe- 

 cies in question, most consistent with f. circumscripta, which it 

 fully resembles both in habit and in structure. The perithallic cells 

 in the type-specimen of L. durum are squarish, 7 — 9 ji in diameter, 

 vertically elongated, up to 14 \j. long and 11 fi broad, or here and 

 there slightly horizontally elongated. It is, however, described as 

 bearing tetrasporic sporangia. As to the occurrence of such ones 

 in the species in question, cp. above. From the north west coast 

 of America I have several specimens of Ph. compactum bearing 

 bisporic sporangia, which seems to show that it also there is 

 usually bearing such ones. On the contrary, Ph. loculosum, 

 which is nearly related, is generally carrying tetrasporic sporangia. 

 Cp. below. 



Although Ph. compactum is a characteristic species and well 

 defined, it can be confounded with some other species. Thus it 

 sometimes bears in habit a great resemblance to Lithophyllum 

 incrustans, particularly when it carries irregular or angular excres- 

 cences. Besides young or even fertile specimens of the species 

 are easily confounded with Lithoph. orbiculatum. As a matter 

 of fact, when the roof of the conceptacles is decorticated, it often 

 happens that first only one pore becomes visible, afterwards more, 

 and if the conceptacles are at the same time feebly developed and 

 smaller than usual, it will be almost impossible to distinguish 

 particularly stunted specimens from the species last mentioned or 

 from a stunted Phym. polymorphum. 



Ph. compactum, however, is particularly closely connected 

 with Ph. loculosum (Lithoth. loculosum Kjellm.), which was ori- 

 ginally found in the Behring's Sea. 2 ) Specimens of the latter spe- 

 cies which are not too old are provided with similar striae as in 



v ) If this species is kept as an independent one, it is to be denominated, 

 consistently with the other species of this subgenus: 

 Pliymatolithon (Clatliromorphum) evanescens Fosl. 

 -) F. R. Kjellm an. Om Betinghafvets Algflora. Stockholm 1889. Pag. 21. 



