126 CARBONIFEROUS CORALS AND BRACHIOPODS 
fact that, in this species, the hinge line is normally less 
than the width of the shell makes it easy to distinguish 
from our form by a comparison of the growth lines, whilst 
there is no trace in the mutation of the fine longitudinal 
stria tion on the ribs which characterizes the foreign species/ 
The smaller specimens closely resemble S. clathratus, 
McCoy, with which they agree exactly in shape, convexity 
and fineness of ribbing as well as in the narrowness and 
depth of the sinus and in the small number of ribs on the 
fold. [That McCoy’s species belongs rather to the group of 
S. hisulcatus than to that of S. striatus has been pointed 
out by Mr. J. Wright (Dav., p. 221)]. Our mutation seems 
to differ, however, in the infrequency of split ribs on the 
fianks ; but though McCoy cites this as a character of his 
species, his figure does not appear to bear him out, as not 
more than two of the fiank ribs show any tendency to split- 
ting. The fact that the fine reticulate striation of the 
ribs is not noticeable on our specimens may easily be due 
to their poor state of preservation. 
McCoy states that his species grows to the size of Spirifer 
giganteiis, a remark which applies equally well to our 
mutation. 
The figured specimen is the ventral valve of a full-grown 
specimen from the Black Rock Quarry ; it shows a trace 
of the side wings which characterized its middle age ; the 
number of split fiank ribs is greater than usual. The figure 
also illustrates how shallow and indistinctly bounded the 
sinus becomes in old age. 
There is one other specimen in the collection. 
Mutation B^ [ ~ part of Spirifer attenuatus, McCoy (non 
Sow),] PI. 2, Figs. 3, 3a, 35. 
This mutation is well characterized by its usually trans- 
1 The relationship of this mutation to Spirifer tornocensis, cle 
Kon, is, for the present, deferred. 
