466 Dr. ri. A. Nicholson and Mr. R. Etheridge on 



than a fourth of a line. Each rosette consists of from ten (some- 

 times fewer) to fifteen or twenty vesicles ; and the free sm-face 

 of each carries a single median row of excessively minute, 

 somewhat slit-like, closely approximated pores. The rosettes 

 are connected together by delicate creeping filaments, which 

 may spring from the bases of the rosettes or from the attenu- 

 ated extremities of the vesicles, and which generally anasto- 

 mose, so as to form a network or mycelium. 



Ohs. In its general structure and arrangement this species 

 is related to A. stellatuni, though sliarply distinguished by the 

 very elongated form of the vesicles and the presence of but a 

 single row of pores on each. All the rosettes, when well 

 preserved, show a circular central cavity or depression, with 

 a distinct bounding wall ; but we have been unable to make 

 out the true nature of this or its relation to the vesicles. 

 When the vesicles are very numerous, they are smaller in size 

 than when the rosette consists of fewer ; but in all cases each 

 shows a dark median line, which, when highly magnified, 

 resolves itself into a line of minute close-set pores (fig. 11). 

 The stolons may ramify and form a network ; or a single stolon, 

 proceeding directly from the end of a vesicle in one rosette, may 

 be prolonged at once into the attenuated termination of a 

 vesicle belonging to another rosette (fig. 10, a). Weathered 

 specimens show clearly that the vesicles are traversed by a 

 long tubular cavity, con'esponding in form with the shape of 

 these structures themselves ; and they sometimes show what 

 appear to be apertures at their bases. The stolons also are, 

 doubtless, tubular, and they probably carry a median row of 

 pores on their free faces, though we have not been able to 

 determine either of these points to our satisfaction. 



There appear to be two well-marked varieties amongst the 

 forms which we have placed under A. radians : — 



Var. a. Vesicles few, lobate, and larger than in h. 



Var. h. Vesicles very numerous, smaller and finer than in 

 a, and the individuals always more crowded together. 



Form, and Loc. Boghead Quarry, near East Kilbride, 

 Lanarkshire, in shale of the Calderwood series, L. Carbonife- 

 rous Limestone group. 



Collected by, and in the cabinet of, Mr. James Bennie, 

 Edinburgh. 



Systematic Position and Affinities. 



After a very careful examination of a considerable number 

 of specimens of the singular organisms which we have grouped 

 together under the name of Ascodictyon^ and after taking the 

 opinion of several of our fellow workers, we are still unable to 



