170 THALLOPHYTA. [CH. 



of a transverse section through the calcareous outer portion 

 of a branch of Cymopolia ; the darker portions or cavities in 

 the calcareous matrix were originally occupied by the lateral 

 branches and sporangia \ 



In Fig. 33, B, the sporangial branch with the terminal 

 sporangium and three of the investing branches are more 

 clearly shown, the surrounding calcareous investment and the 

 thallus having been removed by the action of an acid. 



In a transverse section of a branch from which the organic 

 matter had been removed, and only the calcareous matrix left, 

 one would see a central circular cavity surrounded by a thick 

 calcareous wall perforated by radially disposed canals and con- 

 taining globular cavities; the canals and cavities being occupied 

 in the living plant by branches and sporangia respectively. 



The two circular cavities shown in the figure mark the 

 position of the sporangia which are borne on branches with 

 somewhat swollen tips. From the summit the left-hand 

 sporangial branch shown in fig. 33, A, three of the secondary 

 branches are represented by channels in the calcareous matrix ; 

 the two black dots on the face of the sporangiaphore being the 

 scars of the remaining two secondary branches. 



By the lateral contact of the swollen ends of the ultimate 

 branches enclosing the sporangia the whole surface of the 

 thallus, when examined with a lens, presents a pitted appearance. 

 Each pit or circular depression (fig. 33, N) marks the position 

 of the swollen tip of a branch. 



This form of thallus represents a type which is met with in 

 several members of the Dasycladaceae. It would carry us 

 beyond the limits of a short account to describe additional 

 recent genera which throw light on the numerous fossil species. 

 For further information as to the recent members of the family, 

 the student should refer to Murray's Seaweeds^, and for a more 

 detailed memoir on the group to Wille's recent contribution to 

 the Pflanzenfamilien^ of Engler and Prantl. Among the various 

 special contributions to our knowledge of the Dasycladaceae, 



1 Solms-Laubach (91) p. 38 gives a detailed description with two figures 

 of a recent species of Cymopolia. 



2 Murray G. (95). ^ Wille (97). 



