163 
Relations and Differences. — This Stroplionmies belongs to a small 
number of Carboniferous species, the shells of which are so very like some 
Silurian species that most Pal93ontologists have thought, with a good deal 
of reason, that they Avere produced by the same animal, which had sur- 
vived all the convulsions to Avhich the Palseozic System had been subjected. 
The supporters of this oj^inion, which I also held in 1842, but now believe to 
be erroneous, explain the constant characteristic differences which exist 
between the Silurian and Carboniferous species, by the differences in the 
conditions under which they lived and were developed. I must admit that 
this explanation does not satisfy me now, because, in comparing a large 
number of Silurian specimens from divers countries, from rocks of very 
varied nature, the dcA^elopment of which must necessarily have taken place 
under the most unlike conditions, I have not found any well-marked differ- 
ence, but it has always been easy to refer them quickly to their type. The 
same observation applies to a large collection of Carboniferous specimens 
which, with the least care, could never be confounded with their Silurian 
relatives. The chief differences that have ahvays struck me are these; — 
Pirst, the much greater development of the visceral or reticulated part in the 
S. analoga, and its relatively greater thickness ; secondly, the shape of the 
vascular and muscular processes of the two valves. The muscular process also 
of the ventral valve of S. analoga never has the cordiform appearance which is 
evident in the corresponding valve of S. rhomboidalis, and this last never 
shows any trace of the numerous vascular branches frequently seen on the 
internal casts of S. analoga., and which have been perfectly figured by Mr. 
T. Davidson (^). The muscular process of the dorsal valve is not less different 
in the two species ; in 8. analoga it is narrower, and the principal impres- 
sions of the adductor muscles in it are less developed, and, above all, much 
less rounded than the corresponding ones in 8. rhomboidalis. I grant that these 
characteristics are not always easily seen, and that it is necessary to examine 
a great number of exceptionally well-preserved specimens in order to grasp 
them ; but that is not the point : that these characteristics are constant for 
both forms is enough to convince me that they are different species. Perhaps 
I shall soon have occasion to treat this subject afresh, in which event, being 
supplied with numerous and beautiful specimens, I shall figure and describe 
them with the utmost care. 
(1) Mon. Garb. Brach., pi. 28, fig. 9, 10, 11. 
Z 
