78 



found to deviate, in some measure, from those which I now submit to 

 the Society ; but their deviation of form is to be attributed to acci- 

 dental circumstances, to imperfect or distorted structure. Each of 

 these deviations from the type of the species under one or other of 

 which I have been able to class them, being, so far as regards the de- 

 viation of form, sui generis, differing from all others of the distorted 

 individuals ; so that I have never been able to discover among them 

 all such a general character of form or resemblance as would warrant 

 the classification of them to a separate species. 



The classification hitherto recorded I consider very imperfect. — 

 Professor Ehrenberg does not, I believe, include more than five or six 

 species, and assigns to a distinct species some which 1 deem distort- 

 ed varieties. The want of a nomenclature to characterize those which 

 in my continued observations were so constantly recurring, induced 

 me, for my own convenience (and without the most remote intention of 

 offering the result to this Society) to adopt the arrangement of the spe- 

 cies under the number and nomenclature I now proceed to suggest. 



The eleven species I have arranged in order, as they gradually 

 become more complex in the structure of their tentacula. I am of 

 opinion that all these Xanthidia cannot properly be considered as 

 varieties of one species, varying only in form, according to the diffe- 

 rent stages of development in their progress towards maturity or 

 decay. The species are clearly distinct, as well in their general cha- 

 racter, as in the details of their external structure ; so that mere pro- 

 gress towards maturity or decay will not account for their distinctive 

 peculiarities. Although, it is true, among the individuals of each spe- 

 cies, 1 have observed more or less completeness in their specific de- 

 velopments ; and, as before intimated, these I have deemed merely 

 varieties from the perfect type of their respective species. With these 

 preliminary observations I shall now enumerate and describe the ele- 

 ven species of fossil Xanthidia, giving my reasons, as I proceed, for 

 the nomenclature adopted. 



First, Xanthidium vestitum: which I so name from the thin transpa- 

 rent membrane which extends beyond the body to the extremity of the 

 tentacula, and by which the animalcule appears to be surrounded. It is 

 rarely seen to encompass the animalcule as perfectly as that delineated 

 in the accompanying drawing, the membrane being more or less broken 

 around the outer circumference. This description is of the appear- 

 ance as a superficies or disk, rather than of a solid spherical body, 

 which it really is ; and I cannot help thinking that this appearance 

 results from a gelatinous substance in which the animalcule was. 



