RHODOPHYCE.E, OR FLORIDE^ 241 



and the auxiliary cells (joint cells of ordinary thallus 

 filaments) are associated together in fertile regions 

 of the cortex. The gonimoblasts are very minute 

 and numerous, and produce carpospores in nearly all 

 their cells. The family, though a small one, is of 

 general distribution, and is represented in British 

 seas by the genera Petrocelis, Cruoria, Peyssoncllia, t 

 Hcematocelis, and Rhododermis. 



Comllincce. 



Of all Rhodophycem this family is the most easily 

 recognised, from the strong incrustation of the thallus 

 with carbonate of lime, producing a stony consistency. 

 The different forms are. however, of the most various 

 outward appearance. As in Nitophyllum, so here the 

 thallus filaments undergo subsequent intercalary 

 divisions. Melobesia is of encrusting habit, like some 

 of the Squamariece, growing by. marginal initial cells, 

 at first circular, but afterwards becoming lobed and 

 irregular through unequal development. Litho- 

 phylhim forms thin stony plates of erect habit, while 

 Lithothamnion gives rise to massive stony branches 

 (Fig. 83). Starting from a stone or shell, which the 

 thallus subsequently encloses more or less as a kind of 

 core, its branches frequently form massive structures, 

 in some cases almost rivalling the animal corals in 

 bulk. They occur in particular abundance with the 

 true corals, and the species of Lithothamnion, Melo- 

 besia, &c., often act as a kind of mortar in holding 

 together the reef-building corals. Corallina, Am- 

 phiroa, Jania, and Cheilosporwn, are beautiful, stony, 



R 



