CARBONIFEROUS CORALS AND BRACHIOPODS 125 ' 
groups by strongly marked grooves ; tbe ribs on the flanks 
normally simple throughout their course (i.e. no doubling 
of ribs, either by splitting or by intercallation), with the 
exception of the two ribs on either side of the fold in which 
splitting is the normal rule ; margin of fold truncated. 
In the ventral valve, the sinus is, at first, usually com- 
posed of a single, buried rib with a strongly projecting rib 
on either side ; the number of ribs in the sinus gradually 
increases, with a.ge, by the repeated splitting of these side 
ribs, whilst, at the same time, the boundaries of the sinus 
become less and less distinct. The ribs on the flanks are 
simple and continuous, as in the dorsal valve, but occasional 
splitting is less infrequent than is the case in that valve. 
The hinge line is, as a rule, at least as long as the greatest 
v/idth of the shell. 
By this definition we connote a group rather than define 
a species, for, by variations in the convexity and dimen- 
sional ratio of the valves, as well as by alterations in the 
nature and spacing of the ribs, we can produce an almost 
endless series of distinct forms. Two such mutations occur, 
in enormous abundance, in the lower part of the Carboni- 
ferous Limestone series of the Bristol area, both of which 
are characterized by fine ribbing. These I shall denote 
simply by the letters and 
Mutation B^( =Spirifer ait dathratus, McCoy). PI. 2, 
Fig. 3. 
Ventral valve very convex ; ribs somewhat close together 
and a little flattened ; there is ilsually (especially in middle 
age) a short wing on either side of the hinge line, but the 
general contour of the valve is, in middle age, nearly semi- 
circular, becoming still less transverse when full grown. 
When full grown, this mutation bears a strong general 
resemblance to Spirifer cinctus, A de Keyserling ^ but the 
1 De Kon, Bull, musee Royal Belg., 1883, PL 15, Fig. 4. 
