

334 ZOOPHYTES. 



lichens against a stump. The outer surface in these consists of a 

 series of parallel ridges, transversely striated with very minute 

 lamellae, and having puncture-like oririmes at the bottom of the val- 

 leys, and rarely upon the ridges. The wave-like appearance of the 

 surface suggested the name Undaria, which Oken applied to the 

 group, from unda, a wave; while the mode of growth and general 

 appearance led to Lamarck's appellation Agaricia, from Agaricus, a 

 mushroom. As the plates grow nearly erect, the formation of these 

 ridges is nothing but the extension by growth of the outer or free sides 

 of the seriate polyps, actually nearly horizontal in position. The 

 under or back surface is very finely striated, and nearly flat. These 

 Agariciae are sometimes attached by a margin, and spread only up- 

 ward ; in other instances, they spread in every direction from the 

 point of attachment, but more upward than downward, and thus 

 assume reniform shapes. 



In another group the subgenus Mycedia, the species are some- 

 times unifacial, and nearly resemble the preceding; but they are also 

 often erect and bifacial. The polyps are generally more or less seriate, 

 but many have distinct cells, or fossae consisting of coalescing cells ; 

 the ridges, instead of being evenly transverse, have frequently a reti- 

 culate aspect. The texture of the corallum is peculiar in being very 

 solid. In the species of Mycedia which connect the two groups, the 

 transverse ridges are regular about the centre of the frond, but at the 

 margin they consist distinctly of more or less coalescent cells. The 

 name adopted for them was given by Oken to a genus including a 

 characteristic species ; the word is from JAUXIJ, a fungus. The outer 

 limits of the two groups are very distinct in their appearance, yet 

 they so pass into one another that they are here retained as subgenera 

 of one and the same genus. 



The Mycediae are peculiar to the West Indies, while the Undariae 

 are almost exclusively from the Pacific and Indian Oceans. The 

 former sometimes resemble the Astraeae in their cells, but, as in the 

 Psarnmocorae, these cells arise only from an enlargement outward of 

 the parts of the animals around the mouths. The lamellae are much 

 more minute and crowded than in the true Astraeas, and the animals 

 have no distinct disks, the whole surface being, properly, a single 

 compound disk (as explained in $ 78), which follows all the hills and 

 valleys of the surface. 



The Agariciae resemble the Pavoniae, but the latter have a plane or 



