316 FUCOIDES IN THE COAL FORMATIONS. 



group may casually, under peculiar circumstances of habitat, have their laminas or fronds 

 contorted or twisted around their axis, which is here merely lateral. This twisting occurs 

 in a remarkable degree in many species of our living Algae, especially in those of a hard 

 leathery texture. The most common Fucus vesiculo&us, for example, can be seen around 

 Boston, on grassy meadows submerged at high water, with its fronds so strongly twisted 

 that its length is reduced by one-half, and that it then looks rather like a whorl of leaves 

 surrounding a central axis than like a long flat linear frond, which it is really. If, there- 

 fore, fronds, like the one represented p. 80, fig. 2, of Prof. Hall's report, or plate 1, fig. 2, 

 of ours, whose length attains one foot, were twisted around a lateral axis, here the con- 

 tinuation only of the primary stem, which may force the torsion, the figure resulting from 

 a cross-section of any part of the twisted frond, or from its perpendicular compression, 

 would represent a disk just like that of the new species of Spirophyton. And the same 

 twisted leaves, if compressed in different ways and at various angles, would of course pro- 

 duce multiple deformations like those remarked in the polymorphous Fucoides Cauda- 

 galli. If this supposition is right, and if all the forms under which the Fucoides of this 

 group are seen, may be explained by it, it excludes the necessity of a new genus and pre- 

 vents the scattering of plants of similar characters into different groups. 



5. RELATION OF THESE FUCOIDES TO LIVING ALGJ5. 



It is right to remark, nevertheless, that we have now a genus of living Alga?, repre- 

 sented by one known species only, whose growth seems to be somewhat analogous to the 

 spiral development of the Spirophyton, as it is described by Prof. Hall. It is the Thalas- 

 siophyllum clatlirtis, Post and Pup., growing on the northern shores of the Pacific, in Rus- 

 sian America. According to Mertens, who has described it, the stipe of this plant is very 

 bushy and branching, each branch bearing at its extremity a leaf, which unfolds spirally 

 in such a manner that a spiral border, wound round the stipes, indicates the growth of the 

 frond. This frond presents a large convex bent lamina without nerves, or, to a certain de- 

 gree, a leaf, of which one-half is wanting, for the stipe may be considered as an eccentric 

 nerve.* Though an analogy of development may, from this description, appear to exist 

 between the fossil and these living Alga?, there is, I think, an evident and great difference. 

 In Spirophyton, as it is described and figured, it is not a kind of border or stalk, which 

 causes by its own twisting the bend of the frond ; it is the lamina which unfolds itself in 

 spiral from its point of attachment and expands in ascending. Hence, fronds of this kind 

 can be but simple, while the Northern Alga? of the Pacific are remarkably bushy branch- 

 ing. These, moreover, belong to a class of highly organized Alga?, while in early geo- 

 logical ages and from analogy with what we know of other beings, we can look in the 

 vegetable world for types only of a very simple organization. 



* T. H. Harvey's Nereis Boreali Americana, vol. i, p. 97. 



