McLean. — On the Fossil Genus Sporocarpon. 73 
allied in both directions to Traquairidae and Radiolaria, and probably 
affording links of connexion between the two. It may be regarded as 
a parallel development to the Traquairidae, but with a mineralized skeleton. 
There remain then only the genera Sporocarpon and Oidospora , which it is 
here proposed to treat as one, giving a more detailed account of the species, 
with more complete illustration than was possible in the preliminary 
treatment, and sketching the probable lines of affinity between them and 
living Rhizopoda. 
Sporocarpon. Williamson. 
1878. Williamson, On the Organization of the Fossil Plants of the 
Coal Measures. Part IX. ‘Phil. Trans.’, pp. 169-346. 
The delimitation of the genus cannot be regarded as satisfactory, even 
for a fossil, inasmuch as it is scarcely possible to frame a definition which 
will reduce the divergent structures to a common denominator. Strictly 
speaking, almost all the six or more ‘ species ’ should be segregated as 
distinct generic types, and were it not for the fact that each genus so 
constituted would be practically monotypic this should be done now. 
Should it become possible in the future to distinguish more than one type 
within the limits of what is now called one species, as has been done in 
Traquairia , then the larger species thus subdivided will inevitably have to 
receive new generic titles. For the present, however, there is much to be 
said in favour of retaining the old name Sporocarpon to cover all the forms, 
since all are obviously closely connected, and, in the absence of fuller 
knowledge, too fine a subdivision would unquestionably be valueless. It 
should be clearly laid down at the start that Sporocarpon is no true genus, 
but a plexus of somewhat heterogeneous ingredients, imperfectly separable 
from one another and only referable in one or two particulars to a common 
type. The error of nomenclature is even greater in this case than in fossil 
plants in general, since so much depends, in lower organisms, on cytological 
details of the plasma, here no longer available as criteria. 
The genus Oidospora , founded by Williamson at the same time as 
Sporocarpon , differs from the latter only in its smaller size. Indeed, it is 
not improbably merely a juvenile stage of X. conipactum , though this 
cannot be regarded as established. It is monotypic, and it does not seem 
possible to frame any definition of the larger genus which will exclude the 
smaller, save only on the point of size, which is, on the other hand, so 
variable within the genus Sporocarpon , as at present constituted, as to make 
it unwise to lay stress upon it as a generic distinction. Considering this 
close approximation I propose to suppress its generic isolation and to treat it 
here as a species of the Sporocarpon group under the name of X. Oidospora . 
To define the limits of Sporocarpon brings us against considerable 
difficulties. In my first account of the Traquairidae I gave a short 
