232 OUR COMMON BRITISH FOSSILS. 



see it in every museum, and in most private col- 

 lections ; and it can be bought very cheaply, which is 

 perhaps a still better proof of its commonness. This 

 Brachiopod can be gathered by handsfull in the reefs 

 of the Australian coasts. Evidently, in the "good 

 old times " (which must be the last lingering memory 

 of the Brachiopods), this habit of living together not 

 only characterized the race, but had very important 

 geological results. Who can visit the bold Eglwysey 

 rocks, in the Vale of Llangollen, for instance, and 

 see how the white limestone is in places composed 

 entirely of the valves Prodiictus Llangolliensis, with- 

 out feeling that if it had not been for the abundance 

 and gregarious habits of this Brachiopod in the early 

 Carboniferous seas, those very rocks would never 

 have been in existence ? The same may be said of 

 the Carboniferous limestone near Buxton, where the 

 huge Productus giganteus — not unfrequently as large 

 as a child's head — builds up the rock wholesale. 

 Rhyitchonella Wilsonii in the Silurian limestones, and 

 Rhynchonella socialis (deservedly so called) in the 

 Oolites, have performed the same geological feat. 



The ProductidcE are very easy to make out, not- 

 withstanding their strong specific differentiations. The 

 two shells are either concavo-convex, as in Productus 

 giganteus, P. Martini, P. cora, etc., or round above 

 and flat beneath, as in Productus punctatus, etc. The 

 valves are, moreover, frequently adorned with spines, 

 as in Productus spinosus (armed with them like a 



