CORAL ALCxAE 201 



systematic position by the skilled examination of the expert 

 in the group. 



Melobesia. — The genus Melobesia consists of a number 

 of species which are usually found encrusting rocks or stones 

 or epiphytic on other Algae. They consist of thin plates 

 frequently round in outline, following closely the form of 

 their support and often fusing laterally with neighbouring 

 thalli to form continuous plates of considerable extent. At 

 the surface there may be seen prominent rounded or conical 

 protuberances which contain the conceptacles, and these are 

 perforated when ripe by a single median aperture. The 

 greater part of the thallus of Melobesia is only one layer of 

 cells in thickness, and as the members of this genus do not 

 increase in size vertically they never form thick massive 

 structures. It is only in the regions of the conceptacles that 

 the thallus is more than one layer in thickness. 



There is one other character of importance that is of 

 assistance in the recognition of Melobesia, and that is the 

 presence of small hair-like processes which project from 

 the surface of the ordinary {i.e. not conceptacular) parts 

 of the thallus, giving the surface a somewhat velvety 

 texture. 



This last character separates the genus Melobesia from 

 Heteroderma, which in other respects it closely resembles. 



There are about sixty species of Melobesia and Hetero- 

 derma widely distributed and often verv abundant in the 

 tropical and temperate seas of the world. 



LiTHOTHAMNiON. — The geuus Lithothamnion is even 

 more widely distributed and abundant, and the numerous 

 species exhibit an immense variety of form and structure, 

 some being encrusting plates, others forming papillate 

 clumps or free knolls, and others again growing into small 

 branching shrubs. 



This genus can usually be distinguished from Melobesia 

 by the thickness of the thallus, which always consists of 

 several layers of cells, but confusion may arise between larger 

 specimens of Melobesia and young Lithothamnions unless a 

 critical examination of the microscopic structure is made. 

 The distinction between Lithothamnion and Lithophyllum 



