190 BRITISH AVONIAN CONODONT FAUNAS 
DeEscrIpTION. The platform is lanceolate, from two to three times as long as wide, 
being widest at mid-point, narrowing considerably both to the anterior and to the 
posterior. The platform ornament consists of a fairly strong carina, often extended 
beyond the platform as a short posterior blade, the nodes being slightly higher in this 
region. There is an unornamented trough on either side of the carina, accentuated by 
the upturning of the platform edges, especially in the anterior. The upturned 
platform edges bear short transverse ridges, terminating in denticles, which give a 
crenulate platform edge. The ornament varies considerably ; the juveniles tend 
to be smooth, and the strength of the ridges varies in the adults. The anterior 
blade is fairly long and high, consisting of 7 to 8 tall, laterally compressed, fused 
denticles, which are highest at the anterior end and decrease regularly in height 
towards the junction with the platform. 
In aboral view the unit is very distinctive, the basal cavity being large and 
elongate. It tends to close slightly with age, but is always extremely large, being 
widest at the anterior, and narrowing gradually towards the posterior, where a short 
grooved keel is present. 
REMARKS. This subspecies is distinguished by the large basal cavity. Forms 
described as P. marginatus Branson & Mehl, by Rexroad & Scott (1964 : 37) are 
recorded as having large basal cavities and may be referable to P. Jacinatus s.s., but 
the lack of an aboral illustration makes us hesitate to include them in the present 
synonomy. Orally this subspecies is highly variable, and some forms might be 
mistaken for P. inornatus. The most distinctive feature is, however, the form of the 
basal cavity. In this, apart from the length of the basal cavity proper, the most 
conspicuous feature is the very strong lateral and longitudinal extension of the lips 
around the cavity. These extend posteriorly to the cavity, and become obsolescent 
only in the posterior quarter of the unit, although they narrow posteriorly towards 
that point from their maximum at the posterior termination of the cavity. There is 
no chine-like structure on the aboral surface, and the low-angle sloping outer aboral 
margins of the platform join the lateral lips of the cavity at a sharp angle. Most 
specimens tend to show an almost siphonodellid development of the anterior end of 
the platform, though this itself shows some variation. The Siphonodella-like spout 
of P. lacinatus prelobatus is even more strongly developed than that of the present 
subspecies. The strongly developed posterior aboral extension of the carina, and 
the deep anterior blade give a distinctive appearance to the platform in lateral view. 
It tends to “ ride high” on the surface of the lateral blade. 
Polygnathus lacinatus prelobatus subsp. nov. 
Plate 11, figs. 5a-7b, I1a—c 
DERIVATION OF NAME. From the lobate nature of the inner platform. 
DiaGnosis. Subspecies of P. lacinatus with lobate postero-inner platform. 
MATERIAL. 179 specimens: Holotype X 371, Paratypes X 372, X 373, X 374 
(all figured). 
