506 2'he Carboniferous Succession beloiv the Coal-Mcamres. 



are characterised by cardinal teeth and a second pair of adductor 

 impressions in the ventral valve. The otlier characters are like 

 those of Productus." 



He has omitted two very important characters which ai'e present 

 in the former species. These are a well-marked area and a hollow 

 beneath the beak of the brachial valve, two characters which in 

 addition separate the species from Productus. The extraordinary 

 thickness of the brachial valve of this species is very remarkable, 

 reaching in some cases to one inch. 



Productus gigantens. A good deal of detailed work requires to be 

 done on the sliells which come under this group name. It is 

 questionable whether all really, sensu stricto, can come into the 

 genus Productus. In scune specimens there is a distinct area and 

 other characters which point rather to an affinity with Chonetes. 

 We liave mentioned in the text that the group which we temporarily 

 allude to as F. gigantens lived together, and that the variant forma 

 or mutations have no value as an index of time. 



Productus autiqudtus. A very elegant reticulate form belonging 

 to this genus is not uncommon throughout the Dibunophyllnm zone 

 of North Wales. We know it from the Midlands at the same 

 horizon. It is distinct enough to deserve a specific name, which 

 will doubtless be given to it when the genus is revised. 



Productus Cora. We have retained this name for the present, on 

 the statement of de Koninck that D'Orbigny's type from Bolivia was 

 identical with the Belgian species, although neither the original 

 description nor figure is good. 



The careful search for fossils in the North Welsh Carboniferous 

 rocks has yielded no new forms, but certain details of tbe fauna are 

 worth recording. 



In the limestone the number of species of each group of organisms, 

 except the corals, is much less than are present at the same horizons 

 in the Derbyshire and Staftord^hire inlier of Carboniferous Limestone. 

 Of the Molliisca the Cephalopoda are only represented by about 

 half a dozen species, and individuals are extremely rai'e. Both 

 Gasteropods and Lamellibranchs are comparatively as rare as the 

 Cephalopods. Morton quotes about 30 of each family. The 

 Brachiopoda. as might be expected, are more common, and are 

 represented by about 50 species. In this group individuals are 

 frequent, but not so common as in the Midlands. 



The only species which occurs in North Wales and has not yet 

 been found in North Stalibrdshire or Derbyshire is Allortsmo maxima, 

 Portl., sp., the Sanguiuolites clavn of Morton's list. In the paucity 

 of species and individuals North Wales agrees with the Bristol- 

 Meudip area. Conditions, therefore, must have been very similar. 



P. memhranncea has a very limited vertical, but a wide horizontal, 

 range. It is always found in blac-k shale or dark limestone 

 immeiliately succeeding beds with a Visean fauna. It has been 

 found in this position from the west coast ot County Clare to Clavier 

 in Belgium, and it must therefore be regarded as a definite zonal 

 index of the lower part of the Pendleside Series. 



