214 
SULCOCAVA. 
Figuee. 
Fig. 23. Side of a branch showing transition from zooecia with 
suhoral depressions to zooecia with only a slight widening of the 
suture; X 12 dia. Middle Chalk: Chatham. Gamble Coll. 
D. 3972. 
Affinities. 
This ‘ species ’ may be, as Pergens has suggested, only a form of 
S. cristata ; but the series of specimens in the Museum collection 
does not give much support to that idea, and certainly does not 
prove it. The main differences between the two ‘ species ’ are that 
in S. gracilis the stems are less compressed, the apertures are 
smaller and round, and occur in more regular transverse rows. 
The specimen represented by Fig. 23 suggests that the depressif)n 
in the front wall of typical zooecia of Sulcocava are due to enlarge- 
ments of the interzooecial suture. In the lower part of the 
Fig. 23. — Sulcocava gracilis (Orb.), 
specimen the depressions are represented only by widening of the 
suture ; in the upper part the depressions are immediately below 
the aperture and appear as if in the front wall of the zooecia, but 
this is probably due to the backward course of the zooecia, the 
peristomes being obversely bent at their distal ends. 
The figure in d’Orbigny is probably imperfect, his specimen 
having been much worn, so that the determination of the British 
specimens is somewhat doubtful. 
LIST OF SPECIMENS. 
D. 3972. Two fragments (on slide). Middle Chalk. Chatham. Gamble Coll. 
Figd. No. 23. 
D. 3970. Three fragments (on slide). Middle Chalk. Chatham. Gamble Coll. 
D. 428. One young and one old fragment (on slide). Middle Chalk. Chatham. 
Gamble Coll. 
