ALGyE. 984I 



and impressions in rocks of all ages, and especially in the older 

 strata of the earth's crust, have been regarded by palaeontologists as 

 the remains of Sea-weeds. The characters and probable mode of 

 origin of many of these so-called " Fucoids " have been already dis- 

 cussed in a general way (see p. 483 and pp. 148 1, 1482). It is suffi- 

 cient to say here that many of the fossils in question may be safely 

 regarded as not belonging to the vegetable kingdom, but as being 

 rather of the nature of the tracks and trails of Worms or Molluscs, or 

 the burrows of Annelides or Crustaceans ; while others are of purely 

 inorganic origin. Some " Fucoids," on the other hand, such, for 

 example, as some of the Tertiary or Secondary fossils referred to 

 Chondrites, may really be the remains of Sea-weeds. Even, however, 

 in cases where there is a reasonable probability that these so-called 

 " Fucoids " are really Sea-weeds, it is nevertheless impossible to de- 

 termine definitely to what group of the Alga they belong, since the 

 fructification of the plant is not known. In the following brief 

 account, therefore, of the past history of the Alga, only those groups 

 will be treated of which are of palaeontological importance. 



Class I. Diatomace^e. — The Diatoms are unicellular Alga, in 

 which the cell-wall is hardened by the deposition of silica so as to 

 give rise to a glassy case or " frustule," composed of two halves 

 which fit into one another, and which are often minutely sculptured 

 with lines or dots (fig. 1359). The cells may be solitary, or they 



ifilil Wmmm Iggiggg pnpH 



Fig. 1359. — Types of Diatoms, greatly magnified. a, Navicula, from the Kieselguhr of 

 Franzenbad ; b, Actinoptychus, from Richmond, Virginia ; c, Pmnularia, from Santafiora, 

 Italy ; d, Achnanthes, from Degernfors, Sweden ; e, Diaioma, recent ; /, Triceratium, from 

 the guano of Saldanha Bay, Africa. 



may be organically united in rows. Reproduction takes place by 

 the longitudinal fission of the cell, each of the daughter-cells carry- 

 ing off half of the original cell-wall, and producing the missing half 

 afresh, the two halves of the siliceous case being thus of different 

 ages. 



The existing forms of the Diatoms are exceedingly numerous, and 

 have an extraordinarily wide distribution. A few live in moist 

 situations, but the majority are truly aquatic, inhabiting fresh, brack- 

 ish, or salt waters, peculiar types usually being confined to a special 

 habitat. By the accumulation of their flinty envelopes the Diatoms 



