112 
M. T. WARNE 
Fig 3. A, B, Anchistrocheles praebensoni sp. nov., Fyansford Formation (Balcombian), Batesford Limestone 
Quarry. A, RV internal, NMV P122210, paratype, male, x 86. B, LV internal, NMV P122209, holotype female 
x 90. 
Additional material Fifty-one articulated and 
disarticulated adult and juvenile valves from the 
Batesford Limestone and Fyansford and Sherwood 
Formations. 
Dimensions. Holotype, LV, female, NMV PI22209: 
L = 0.73 mm, H = 0.41 mm. Paratype, RV, male, 
NMV P122210: L = 0.74 mm, H = 0.34 mm. 
Diagnosis. Carapace of varying shell thickness, 
with narrow, differentially compressed zone 
adjacent to anteroventral and posteroventral 
margins; very broad inner lamella evident in all 
but a relatively small region adjacent to mid¬ 
dorsal margin. 
Description. Carapace of moderate size, smooth. 
Most adult specimens of medium shell thickness 
(opaque), but rare adults and most juveniles 
with thinner shells (translucent). LV larger than 
RV and strongly overlapping it. LV asymmetri¬ 
cally rounded anteriorly, with convex antero- 
dorsal and anteroventral margins; extremity at 
mid-height. Posteriorly, LV more acutely 
rounded with convex posterodorsal and poste¬ 
roventral margins; extremity below mid-height. 
RV similar to LV anteriorly and posteriorly but 
with straight anterodorsal margin and anterior 
extremity slightly above mid-height. Ventral 
margins of both valves strongly incurved. Dorsal 
margin gently arched in LV, straight in RV. 
Maximum length below mid-height; maximum 
height slightly anterior to mid-length. Normal 
pore canals simple, without rims. Inner lamellae 
with inner and outer margins not parallel. Inner 
margin sinuosity as follows. 1. Anterodorsal part 
of inner margin more or less straight in both 
valves except near its dorsal margin terminus 
where it arches slightly towards valve perimeter. 
2. Anteroventral part of inner margin gently and 
evenly arching towards valve perimeter. 3. 
Anterior section of inner margin approximately 
parallel to outer margin. 4. Ventral and poste¬ 
roventral parts of inner margin with no distinct 
change in curvature between them; they are 
straight in LV and slightly arched towards ven¬ 
tral perimeter in RV, except posteriorly where 
they become more strongly arched as part of the 
acutely rounded posterior extremity of the inner 
margin. 5. Posterodorsal part of inner margin 
more or less straight in LV. broadly arched 
towards centre of carapace in RV. 6. Ventral and 
posterior sections of inner margin not parallel to 
outer margin. Vestibula distinct but relatively 
small. Marginal pore canals abundant and 
straight. Hinge short and adont. Adductor mus¬ 
cle scar pattern comprising four undivided and 
unsutured elliptical scars in a loosely packed 
aggregate. Outer lamellae granular internally in 
thicker-shelled adults, smooth in thinner-shelled 
adults and in juveniles. Sexual dimorphism evi¬ 
dent, males more elongate than females. 
Remarks. Anchistrocheles praebensoni sp. nov.. 
A. bensoni Maddocks, 1969 and A, fuinata 
Brady, 1890 are very' similar morphologically, 
and are here considered to define the morpho¬ 
logical limits of Anchistrocheles s.s. The three 
species can be distinguished from each other by 
their different lateral outlines and by the size 
and sinuosity of the inner margin. 
Specimens identified by Yassini & Jones 
(198 7) as Bythocypris reniformis Brady, 1 880are 
very similar in shape to that species but differ in 
being differentially compressed along a narrow 
zone adjacent to the anteroventral and postero- 
