CARBONIFEROUS CORALS AND BRACHIOPODS 12^ 
The presence of spiral arms is clearly illustrated by twO' 
polished sections in the collection, and is exhibited by hun- 
dreds of chance sections and by weathered specimens in 
the rocks themselves, throughout the Bristol area. It- 
seems indeed most probable, judging by the great similarity 
of form to our specimens, that both McCoy’s species are 
true athyrids (i.e. really possessed spiral arms), notwith- 
standing the fact that both he and Davidson fix upon 
D. hastata as their closest relative. However this may be, 
there is no doubt that the myriads of individuals which 
teem in the middle beds of the Carboniferous Limestone 
series, throughout the Bristol area, are true athyrids, whilst 
possessing the form shown in Figs. 12, 13, 14, 15, PL L, Dav. 
Probably the commonest form is one like PL 1., Fig. 14, 
Dav., but less elongate ; there is, however, every possible 
gradation of shape within the general limits laid down in 
the definition. 
Some mutations closely resemble the figures of Seminula 
subtilita, Hall, but, in our species, there is seldom any marked 
deflection of the frontal margin, whilst the foramen is usually 
larger. 
Another common mutation, in which the athyrid charac- 
^ ters are more prominent and the relationship to ambigua, 
Sow, somewhat strongly suggested, is represented in our 
figures. 
The figured specimen is labelled “ Middle Limestones.’^ 
Nearly all the examples in the collection are from the 
Great Quarry Middle Limestones ”), and include very 
characteristic slabs of shale, from the lower beds of the 
quarry, crowded with crushed specimens. Two polished 
slabs of limestone, labelled “ Terebratula Bed, 2nd Tunnel,” 
exhibit good sections of the spiral arms. 
There are several small specimens from Weston-super- 
Mare, some of which are apparently the very young stage 
of this species ; others are more transverse, with a more 
