102 



THE GEOLOGIST. 



dtictidce, and were always absent in the Strophomenidod ; b.ut 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 Orthis, 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 

 Producta 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 imi^ressions, which are present, although not always clearly 

 distinguishable, in the interior of the smaller or dorsal valve of all the 

 species of Productidce hitherto discovered, but of which no trace has 

 been seen in any of the Strophomenidce at present known. 



The Produgtid^ have been divided into four genera, or sub-genera, 

 viz. Producta, Aulosteges, Strophalosia, 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 Aidosteges, 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 resu^miata and 0. Kerser- 

 lingiana were furnished with small scattered spines. Probably such spiues liave 

 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, but oftener, possibly, through the hardness of the limestone-matrix which 

 adheres to tlie outer surfoce, eausmg the latter usually to flake oft' in breaking 

 out the specimens, and which remains in the mould undetached from the em- 

 bedded spines. For the opposite reason, probably, it is that specimens from shales 

 are generally more perfect than those fi'om limestone^. 



