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FROM NORTHWESTERN EUROPE 67 
carefully prepared material. The ribs maintain a consistent spacing over the 
surface by constant dichotomy. 
The shell is biconvex with the brachial valve the more inflated of the two. The 
beak is small and suberect. Disjunct trigonal deltidial plates flank the elliptical 
submesothyridid pedicle opening. Typically there are one or two well developed 
growth lamellae, although the “very numerous fine growth lines’”’, noted by 
Buckman (1918) have not been observed on the material studied. The shell being 
very thin and delicate, short, slightly diverging dental lamellae and a short median 
septum are frequently seen, in pedicle and brachial valves respectively, of worn 
specimens. 
‘ 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
I-40 cm 0-68 cm 1-55 cm 
3°05 cm 2°20 cm 3°53 cm 
2°46 cm 1-73, cm 2°66 cm 
Internal characters. Pedicle valve. The deltidial plates are only poorly devel- 
oped, as seen externally, but in serial section they are seen to line the subquadrate 
delthyrial cavity laterally. Near the umbo, sections I-o to 1-2 in text-fig. 24 are 
seen two thin concave lamellae forming a weak pedicle collar. The dental lamellae 
are quite strong and parallel to one another, not breaking away from the ventral 
shell wall, as seen in serial section, until the teeth are fully inserted in their sockets ; 
the lateral cavities are always relatively small. 
Brachial valve. There is a long, low median septum. Septalial plates are not 
developed but there is a very distinctive thickening of the inner ends of the hinge 
plates, as shown in plate 12. This thickening possibly served for anchoring the 
didductor muscles. Lateral denticulae are present but, in common with the inner 
and outer socket ridges, are not well differentiated. The crura seem to be quite 
close to the shape described by Muir-Wood (1934) as calcarifer. 
DisTRIBUTION. The author has not collected personally any material of this 
species and, therefore, it seems reasonable to quote the distribution given by 
Buckman and Walker (1889, p. 54) in their original description of the species : 
“ This species appears to be confined to the district south of the Mendip Hills. It is 
a rare fossil, and it occurs only in the parkinsom zone. It is, however, widely 
distributed, and has been obtained in Dorset, at Burton Bradstock, Broadwindsor, 
Clifton Maybank, Bradford Abbas, Halfway House and Combe Quarry near Sher- 
bourne. In Somerset, at Misterton, Haselbury and at Crewkerne Station’’. The 
B.M.(N.H.) material, on which this revision is based, confirms this distribution. 
The species has not been recorded in the literature outside Britain and the only 
possible occurrence known to the author is represented by a few poorly preserved 
specimens collected by him from the parkinsoni zone of Normandy. 
OCCURRENCE. No details were given by Buckman and Walker, but as the species 
seems to be restricted to the Inferior Oolite, it presumably lived in a fairly high- 
energy environment. The material in the collection of the B.M.(N.H.) shows quite 
