INTRODUCTION TO CRYPTOGAMIC BOTANY. 215 



mences at a far lower latitude. It is found, too, in the Pacific 

 and Indian Oceans. The species of Dlctyota, on the contrary, 

 are extremely abundant northwards, being very common on 

 the greater part of our coast; but they are found in far distant 

 realms. HaUseris is a deep sea genus, as far as our own 

 Nereis is concerned, but there are many representatives in 

 warmer climes. The greater part of the species have a decided 

 midrib, which gives them a peculiar Fucoid appearance, 

 very different from that of their allies. Numerous genera are 

 described, most of which are represented on our coasts. The 

 first, however, to be noticed, Hydrodathrus, though occurring 

 in the Mediterranean, is wholly unknown in the British Isles. 

 The fronds, which are very irregular in outline, form wide 

 spreading patches, adhering by their lower surface, and 

 laterally confluent. The young fronds are pierced with round 

 holes of small size, and somewhat pressed together; but as the 

 membrane expands, the holes expand also, and new ones are 

 formed in the interspaces, until the whole membranous wall 

 of the baglike body is converted into a delicate lacework, 

 the margin of each hole being involute, so as to look like a dis- 

 tinct rim. It is found in various localities, extending as far 

 south as Maiaitius and New Holland, whence I have speci- 

 mens. Dr. Harvey's figure resembles closely a group of Cyt- 

 taria, a fungus to which we shall have occasion to advert 

 hereafter. Asperococcus and Punctaria are, to these Algre, 

 what the larger Enteromorphce and Ulvw are to Chlorosperms, 

 being yellow-brown sacs or laminee studded with fruit, while 

 Striaria and Dictyosi'phon represent the branched and almost 

 filiform Enteromorphw. With Stilophora, we arrive at species 

 with a solid or imperfectly tubular frond ; Dictyota commences 

 the Zonarioid group. The structure of this common genus 

 is very curious. " Every lacinia of the frond terminates in a 

 single cell, by the constant division of which at the lower side 

 the other cells of the frond are formed, the terminal cell being 

 thus continually pushed onwards" (a mode of gro-\vtli precisely 

 like that which obtains in the increase of exogenous stems). 

 " Hence it results that the longitudinal lines of superficial 

 cells, which in the flabellate genera diverge from one another, 



