BRITISH AVONIAN CONODONT FAUNAS TRE 
lateral edges. The basal cavity is small and restricted to the aboral surface 
of the apical denticle, but aboral grooves are developed on both the anterior 
and the posterior bars. There is a short stout denticulated posterior bar, 
which bears a series of discrete denticles. 
TYPE SPECIES. Prioniodus angulatus Hinde 1879. 
2. ROUNDYA conodonts of similar overall form to Hibbardella s.s. but charac- 
terized by distinctive basal cavities and by the form of the apical denticle. 
The basal cavity is relatively much larger than that of Hibbardella, although 
it is still confined to the lower surface of the apical denticle. The whole 
aboral surface of that denticle is hollowed out, but the cavity does not flare 
beyond its basal area. The denticle itself is characterized by its distinctive 
cross-section. In its lower half it is broadly sub-circular and robust in form, 
the posterior surface having a longitudinal broad concave cavity which is 
deepest at the proximal end, and decreases in depth towards the mid-length 
of the denticle. The lateral faces of the apical denticle are smooth and 
strongly convex, but in their medial areas a conspicuous groove is developed 
longitudinally in the lower part. From this a strong lateral costa develops 
on each lateral face. In the distal half of the apical denticle it becomes 
strongly anterio-posteriorly compressed, but the lateral edges remain sharp, 
and the anterior and posterior faces broadly convex. 
TyPE SPECIES. Roundya barnettana Hass 1953. 
3. HASSOGNATHUS subgen. nov. conodonts whose general form is similar to 
Hibbardella, but which are characterized by the fact that the anterior bars 
are essentially unexcavated, their aboral surfaces being shallow, concave, 
depressions. The basal cavity is conspicuous, but is largely developed 
below the posterior bar, although it extends and is continuous with one 
below the apical denticle. The apical denticle is usually sub-circular in 
cross-section and unornamented. The whole appearance of members of 
this subgenus is of a Ligonodina which has developed an additional lateral 
bar. 
TYPE SPECIES. Tvichognathus separata Branson & Mehl 1934. 
Subgenus HIBBARDELLA (HIBBARDELLA) Ulrich & Bassler 1926 
Hibbardella (Hibbardella) abnormis Branson & Mehl 
Plates3rjai¢. 6 
1940 Hubbardella abnormis Branson & Mehl: 184, Pl. 6, fig. 14. 
1963 Hibbardella abnormis Branson & Mehl ; Rexroad & Collinson 10, Pl. 2, figs. 15, 18, 20, 21. 
1965 Hibbardella abnormis Branson & Mehl ; Rexroad & Collinson : 9, Pl. 1, figs. 8, 9. 
