132 CARBONIFEROUS CORALS AND BRACHIOPODS 
at which intermediate ribs start and the rapidity with which 
these intermediate ribs acquire the size of the primary 
series. I defer the necessary discussion of the relationship 
of these forms to wrinkled examples of Productus hemi^ 
sphericus, Sow, with which they are very usually confused. 
Producti of the giganteus group. 
The specimens in the collection being very few and very 
poor, there is no need, in this place, to enter upon the most 
difficult of all the questions which cry for solution, viz. the 
relationship of the various mutations, at present included 
under the loose specific name giganteus, to each other and 
to nearly related forms. 
Productus scabriculus, Martin. 
Fragmentary specimens, probably belonging to this 
species, from beds usually classed as Millstone Grit, are 
represented in this collection. They have been derived 
from grit beds “ Behind the General Draper,” i.e. above the 
top of the recognized Carboniferous Limestone series. 
Clionetes : 
Chonetes Buchiana, de Kon. 
There are numerous specimens, crowding slabs from the 
well-known horizon in the “ Lower Limestone Shales.” 
These have already been figured by Davidson (PL 55, fig. 
12 ). 
Chonetes Hardrensis, Phill. 
Slabs covered with a mutation of this species (near 
Chonetes polita, McCoy) have been derived from the “ Lower 
Limestone Shales ” and from the Black Rock Quarry. 
Chonetes aff. comoides, Sow., and aff. papilionacea, PhilL 
The vast abundance of this form in the rocks themselves, 
and its representation in the collections by a single sped- 
