BRITISH AVONIAN CONODONT FAUNAS 225 
edge. The 3 or 4 anterior denticles are higher than those of the rest of the blade. 
The number of oral denticles is variable but averages 15. The denticles of the oral 
edge are of uniform elevation with the exception of the posterior two or three, which 
may be slightly lower. The basal cavity is situated at the mid-length of the unit and 
is circular in outline. 
The denticles of the oral edge are of fairly uniform elevation, with the exception of 
those at the posterior tip, which are slightly lower, and those at the anterior, which 
rise gradually to form an anterior blade. The anterior and posterior edges are 
straight, and form right angles with the aboral edge. The aboral edge is straight. 
The basal cavity is sub-circular in outline and centrally situated. 
REMARKS. Sphathognathodus coaptus closely resembles S. crassidentatus and S. 
denticulatus. In S. crassidentatus the aboral edge is arched in the posterior two 
thirds. In S. denticulatus the basal cavity is narrow and elongate. 
Spathognathodus costatus costatus (E. R. Branson) 
Plate 3, figs. 13a-15b 
1934 Spathodus costatus E. R. Branson : 303, Pl. 27, fig. 13. 
1938 Spathodus costatus E. R. Branson ; Branson & Mehl: 132, 136, Pl. 33, fig. 1. 
1944 Spathodus costatus E. R. Branson ; E. B. Branson: 181, Pl. 32, fig. 1. 
? 1949 Spathognathodus costatus (E. R. Branson) Thomas : 409, 412, Pl. 4, fig. ro. 
non 1956 Spathognathodus costatus (E. R. Branson) Bischoff & Ziegler : 166, Pl. 13, fig. 3. 
non 1957 Spathognathodus costatus (E. R. Branson) Bischoff : 56, Pl. 4, fig. 28. 
non 1957 Spathognathodus costatus (E. R. Branson) ; Ziegler 7m Fligel & Ziegler: Pl. 1, 
figs. 15, 18 only. 
non 1959 Spathognathodus costatus (E. R. Branson) Helms: Pl. 111, figs. 2-4. 
non 1959 Spathognathodus costatus (E. R. Branson) Voges : 297, text-fig. 5, fig. 1. 
non 1959 Spathognathodus cf. costatus (E. R. Branson) Voges : 297, Pl. 34. 
non 1962 Spathognathodus costatus costatus (E. R. Branson) Ziegler : 107, 108, Pl. 14, figs. 1-6, 
8-Io. 
? 1964 Spathognathodus costatus costatus (E. R. Branson) Higgins, Wagner-Gentis & 
Wagner : PI. 5, fig. 21. 
MATERIAL. 170 specimens : figured, X 455, X 166, X 456. 
Rance. North Crop ZLA 2-ZL to, Avon Gorge K 18-Z 28. 
DEscRIPTION. The unit is slightly arched and bowed, being highest at the anterior 
end, although the anterior blade is only about one third as high again as the posterior 
end. The oral profile is sigmoidal, the anterior blade being highest and consisting of 
3 or 4 large denticles, the tallest of which is either the anterior-most or penultimate 
anterior denticle. The denticles of the mid-region are of equal height, and lower than 
the anterior denticles, and the unit shallows posteriorly. Situated on the inner side 
of the unit is a row of peg-like nodes, 5 to 8 in number, which tend to jut out of the 
unit. These nodes may be joined to the main unit by transverse ridges, but these 
ridges lie below the level of the node tips. 
In aboral view the cavity occurs in mid-length, is fairly large and symmetrical, and 
is widest at its mid-point. It extends as a narrow groove to near the posterior 
termination and for a short distance along the anterior blade. 
