A Fossil Alga. 13 



familiar example the case of many ferns in which, owing to the char- 

 acteristic outward forms on the one hand, and the excellence of the 

 impressions on the other, safe determinations of genera have been made. 

 An objection to the application of this method to the Algae might be 

 made at the outset, that it would only lead us back again along the old 

 path encumbered with the debris of Chondrites and the like. I venture to 

 think, however, while agreeing with Nathorst in the main, that he goes 

 too far in denying that this method may be employed with proper safe- 

 guards, such as a consideration of the nature of the bed as obtained from 

 other remains, the perfection of the remains themselves, and the degree 

 of their correspondence with the bodies of living Algse, especially if the 

 group in question be one of steadfast outward form. This latter character 

 is one unfortunately rarely to be met with among Algae. Their extremely 

 plastic bodies are readily moulded by their immediate environment, and 

 systematic literature is loaded with records of growth-forms erroneously 

 described as independent species. 



Of all genera of Algae, one might almost say of plants, there is none 

 which varies through a wider range of outward forms than Cauhrpa ; but 

 the species themselves are, on the other hand, remarkably constant in their 

 characters. It is only here and there that we find one species run into 

 another, as the saying is. The variation is of a noteworthy character, 

 since the outward forms are astonishingly like those of the higher plants, 

 as their names cactoides, taxifolia, Abies marina, ericifolia, cupressoides, 

 tJiujoides, juniperoides, fontinaloides, Selago, Lycopodium^ liypnoides fre- 

 quently show, while their internal structure is one characteristic of the 

 genus in a remarkable degree. The whole body of the plant consists of 

 a single multinucleate cell, and the walls are sustained by an interwoven 

 system of trabeculae or cross beams and struts traversing the interior in 

 all directions, forming a great plexus, but at no point interrupting the 

 continuity of the cell. The Caulerpce form a group of Siphonea, the 

 same order to which belong those other Algae alluded to above, described 

 by Munier Chalmas, and Nematophyciis as well. Nature seems to have 

 shown in this genus the utmost possibilities of the siphoneous thallus in 

 an exhibition of the outward forms of the higher plants a prophecy, 

 as it were, of what was otherwise accomplished by the building up of 

 cellular and vascular tissues. Small wonder, then, if Caulerpa has been 

 chosen by palaeontologists as a convenient limbo for placing all imperfect 

 sorts and conditions of so-called fossil Algae. I have examined nearly 

 every species known to science of those at present existing, and have seen 

 a considerable number of them alive in West Indian waters, and with this 

 aid to judgment I may say that of all the described fossil Caulerpa and 



