No. 3] REMARKS ON NORTHERN LITHOTHAMNIA. 77 



pany with or coalesces with Litlwthamnion glacial e, or one of 

 these species even partly covers the other. Sometimes it also 

 grows together with Ph. compactum. Once I even found all three 

 species coalesced. The form tuberculata here sometimes assumes 

 forms which in habit approach certain forms of L. glaciale so 

 nearly that, when sterile, they cannot possible be defined. Nor 

 does the structure always give a sure point of definition. This 

 is particularly the case when both species are attacked by boring 

 mussels and assume extreme forms. Pit. polymorphum will then 

 even develop short branches irregularly parted. At the southern 

 part of the British Isles or on the Atlantic coast of France the 

 species in question now and then grows in company with Litho- 

 phyllum incrustans, or one species partly covers the other, and 

 the species approach each other in habit. Occasionally the alga 

 grows partly freely, or two crusts, which in growing come in 

 contact, rise against each other, coalesce and continue growing 

 freely, forming higher or lower ridges, which are undulate-lobate 

 or crenate, all but like what is often seen in Lithophyllum in- 

 crustans. 



As to structure the plant is not much varying, though its 

 development is somewhat disturbed by the numerous overgrown 

 conceptacles or by attacks of animals causing new formations of 

 hypothallium, partly fairly regular, partly quite irregular. The 

 hypothallium is formed by dichotome rows of cells, whose lower 

 anticlines either but faintly converge towards the substratum, or 

 not converging run upwards in large curves, — or the rows of 

 cells, particularly in thick superposing crusts, form rather large 

 curves upwards, running into or sending forth the perithallium, 

 and smaller curves downwards towards the substratum. The 

 hypothallic cells are 12 — 16 p or up to 20 p long and 7 — 12 p 

 broad. The perithallium is composed of cells which are partly 

 squarish, 6 — 8 p in diameter, often with a little rounded corners, 

 partly vertically elongated, 7 — 10 p or up to 12 p long and 

 6 — 8 p broad. 



Typically developed conceptacles of sporangia are immersed, 

 becoming cup-shaped or impressed-oblong, frequently with an 



