BRITISH AVONIAN CONODONT FAUNAS 107 
DEscRIPTION. This species, which is well known and widely distributed in both 
Europe and North America, is characterized by a low, small parapet on the inner side 
of the platform, and a few scattered nodes developed on the middle part of the low 
outer side of the platform. The carina tends to be laterally expanded in the posterior 
part of the platform. Our specimens of this species agree closely with those described 
by other authors. 
Gnathodus simplicatus sp. nov. 
Plate 8, figs. 5a-c. Plate 18, figs. 2a—5b 
1957 Spathognathodus subrvectus Holmes ; Fliigel & Ziegler : 53, Pl. 11, fig. 12. 
1960 Spathognathodus strigosus Branson & Mehl ; Dvorak & Freyer : PI. 1, fig. 17. 
DERIVATION OF NAME. From the simple form. 
Diacnosis. A species showing Spfathognathodus—Gnathodus transition, with 
characteristic anterior blade, its highest point being at or near the anterior end, and 
with oral surface sloping regularly towards the posterior end of unit. Platform very 
feebly developed. 
MATERIAL. 105 specimens: Holotype X 89, Paratypes X 88, go, 91, 415 (all 
figured). 
TYPE LOCALITY AND HORIZON. North Crop. Sample ZLA 33. 
RANGE. North Crop ZLA 15-ZLA 33, Avon Gorge Z 33-Z 38. 
DEscrRIPTION. The unit is slightly bowed and slightly arched, highest in the 
anterior quarter, and sloping posteriorly. The anteriormost one to three denticles 
may be slightly shorter than the penultimate denticles, but otherwise the oral 
outline is straight to slightly convex. The denticles, which number 13 to 15, are 
small and laterally compressed with free chevron tips. The basal cavity is situated 
in the posterior half and runs to the posterior termination ; the lips flare over the 
whole length and the cavity widens to the mid-point where it becomes constricted 
and then runs as a narrowing groove towards the anterior. The basal margin is 
lipped and slightly stepped in lateral view. 
RemMARKS. Middle and Upper Devonian forms similar to those described above 
have been named as Spathognathodus bidentatus by Bischoff and Ziegler (1957) and 
Freyer (1961). Bischoff and Ziegler’s specimens have a greater number of denticles 
(about 17 total) than ours, and the cavity is different, not quite reaching the posterior 
end, and being greatly flared anteriorly. Freyer’s specimens have only about Io 
denticles and are much lower towards the posterior. 
The present species is transitional between Spathognathodus and Gnathodus, but is 
included in Guathodus because the basal cavity is developed posteriorly, is longi- 
tudinally extended, and the lips show a tendency to lateral flare. 
There is some similarity between the forms described here and S. cristulus, which is 
considered by American workers to be the basic stock for all spathognathodid 
lineages. It is possible that the two species are the root stocks for successive 
gnathodid developments, the genus Gnathodus being polyphyletic. 
