142 THALLOPHYTA. [CH. VIl] 



animals, such as certain Polyzoa, the flat branching body of 

 various algae is closely simulated, and in other plants, such as 

 the frondose liverworts, the same thalloid and branched form 

 of body is again met with. Some of the much dissected 

 Aphlehia leaves of ferns (e.g. Rhacophyllum species) bear a 

 striking resemblance to fossil algae ; and numerous other 

 examples might be quoted. In palaeobotanical literature we 

 find a host of names, such as Chondrites, Fucoides'^, Caulerpites 

 and others applied to indefinite and indistinct surface markings 

 which happen to resemble in shape certain of the better 

 known genera of recent seaweeds. 



The close parallelism in outward form displayed by different 

 genera and families of algae is in itself sufficient argument 

 against the use of recent generic names for fossils of which 

 the algal nature is often more than doubtful. Were external 

 form to be accepted as a trustworthy guide, in the absence 

 of internal structure and reproductive organs, such a genus 

 as Caulerpa^ would afford material for numerous generic 

 designations. A comparison of the different species of this 

 Siphoneous green alga brings out very clearly the exceedingly 

 protean nature of this interesting genus, and serves as one 

 instance among many of the small taxonomic value which 

 can be attached to external configuration. Caulerpa pusilla 

 Mart, and Her., G. taxifolia (Vahl.), G. plumaris Forsk., 

 G. abies-marina J. Ag., G. ericifolia (Turn.), G. hypnoides 

 (R. Br.), G. cactoides (Turn.), G. scalpelli formis (R. Br.), and 

 others clearly illustrate the almost endless variety of form ex- 

 hibited by the species of a single genus of algae. We constantly 

 find in the several classes of plants a repetition of the same 

 form either in the whole or in the separate members of the 

 vegetative body, and but a slight acquaintance with plant 

 types should lead us to use the test of external resemblance 

 with the greatest possible caution. To emphasize this danger 

 may seem merely the needless reiteration of a self-evident fact, 



^ An American writer has recently discussed tlie literature and history of 

 Fucoides ; he gives a list of 85 species. It is very doubtful if such work as this 

 is worth the labour. (James [93].) 



2 Wille (97) p. 136, also Murray, G. (95) p. 121. 



