CARBONIFEROUS CORALS AND BRACHIOPODS 127 
verse and attenuate form, the lateral valve-margins being 
almost straight, so that there are no side wings. The mesial 
fold and sinus are of the typical Bisulcate type, and are 
always strongly defined ; the ribs are always sharp and very 
distinct. McCoy clearly included in his definition both 
this form and the true Spirifer attenuatus, Sow. The 
latter is easily separated from our mutation by its large 
number of ribs on the mesial fold and by the similarity 
of their nature and grouping to those on the fianks (cf. 
Fig. 12, PL 2, Dav., and Fig. 12, PL 14, de Kon, loc. cit.). 
PL 6, Fig. 18, Dav. apparently represents a form which 
somewhat closely resembles that under description. 
When quite young, this mutation is remarkably trans- 
verse, but, with age, the growth axially outstrips the growth 
transversely, so that we often meet with examples in which 
the axial exceeds the transverse dimension ; such examples 
are often labelled 8. mosquensis, Fisch, though agreeing in 
no other character whatsoever, except the dimensional 
ratio (e.g. 8. mosquensis has a very finely ribbed mesial 
fold, whilst our form has a typically bisulcate fold, etc., etc.) 
Fig. 3a is a solid specimen showing three groups on the 
mesial fold, each group splitting into two. 
Fig. 36 illustrates the type in which there are two groups 
only, and the splitting takes place rather late. Both figures 
show excellently the splitting of the two ribs on either side of 
the fold. 
Fig. 3c shows a characteristic ventral valve. This speci- 
men has already been figured by Stoddart in a serial work, 
the Palseontologia Bristoliensis (a work of which only two 
numbers were ever published) ; he refers it to 8pirifer 
striatus var attenuatus, Sow., and gives an unsatisfactory 
description of extremely wide application. 
There are a large number of specimens in the collection, 
illustrating both mutations ; they are derived from the 
Lower Limestone Shales ” and the Black Bock Quarry. 
