102 
THE GEOLOGIST. 
dmtidcB, and were always absent in the Strophomenidce ; but it is now 
well known that the presence or absence of spines cannot be con- 
sidered a character of importance, since in many families among the 
Mollusca we find genera and species both with, as well as without 
those appendages. 
About a year or two ago. Prof L. de Koninck lent me a British 
specimen of OrlJiis, like the one which he had represented in PI. XIII. 
fig. 8, of his work on Belgian Carboniferous Fossils, as an example 
of Orthis Michelini ; but, as the specimen in question was thickly 
covered with short spines, similar to those that cover the valves of 
Produeta punctata, he subsequently felt uncertain whether it could be 
referred to the first-named species, which he did not believe to have 
been provided with those appendages. Since that period I have 
obtained several examples of the 0. Michelini which evidently 
possessed numerous scattered spines over their external surface ; thus 
proving the correctness of the Professor's original determination.* 
The only general feature of any importance that can be brought 
forward in the separation of the two families is that of the so-termed 
■reniform impressions, which are present, although not always clearly 
distinguishable, in the interior of the smaller or dorsal valve of all the 
species of Productidoe hitherto discovered, but of which no trace has 
been seen in any of the Strophomenidce at present known. 
The Productid^ have been divided into four genera, or sub-genera, 
viz. Produeta, Aulosteges, Sfrophalosia, and Chonetes ; but, as they all 
bear so natural, and, indeed, so intimate a relation towards each 
other, we will mention the characters of the group, and at the same 
time specify those details by which each in particular has been dis- 
tinguished. I should likewise wish to observe, that a prolonged 
study of the family has disposed me to believe Aulosteges, Strophalosia, 
and Chonetes to be simple sub-genera, or modifications, of Productus ; 
and this is also the opinion of Prof de Koninck. 
* It is likewise certain that some examples of Orthis resupinata and 0. Kerser- 
lingiana were furnished with small scattered spines. Probably such spines have 
escaped observation, not merely from their being of a very delicate nature, and 
consequently liable to abrasion and injury before being buried up in the ancient 
mud, out oftener, possibly, through the hardness of the limestone-matrix which 
adheres to the outer surface, causmg the latter usually to flake off in breaking 
out the specimens, and which remams in the mould undetached from the em- 
bedded ^mes. For the opposite reason, probably, it is that specimens from .shales 
are generally more perfect than those from limestones. 
