138 BRITISH AVONIAN CONODONT FAUNAS 
developed, which is the main anterior edge of the denticle ; it extends aborally as a 
sharp ridge, which forms the edges of the denticles of the anterior aboral process. 
The posterior face of the main denticle tends to become rather depressed and may 
develop a low concave depression. 
The antero-aboral process points vertically downwards, making an angle of 
80°—r100° with the posterior bar. It is not deflected out of line with the main denticle 
to any great extent. The oral surface of the anterior aboral process bears up to 6 
denticles, the largest of which occurs in the medial portion ; the denticles are 
strongly compressed anteriorly and posteriorly in their distal portions. Though 
their bases are sub-circular in outline, they have strong lateral edges and are recurved 
inward and upward ; they are discrete but relatively closely spaced. The posterior 
bar is relatively slender and bears at least 2 (the present specimens are all broken) 
discrete, sub-circular, widely spaced, feebly posteriorly inclined, bluntly pointed, 
peg-like denticles. 
The outer lateral face of the main denticle is strongly flared and the edges of the 
cavity are thick ; there is a deep, but limited lip below the main denticle which is 
extended along the aboral anterior process as a conspicuous longitudinal groove on 
the aboral surface ; it is also extended for at least some distance along the posterior 
bar. 
The width and depth on the antero-aboral process decrease towards the distal 
extremity, which is bluntly rounded. The posterior bar is more or less quadrate in 
cross-section. 
Ligonodina tenuis Branson & Mehl 
Plate 31, figs. 4, 16 
1941 Ligonodina tenuis Branson & Mehl: 170, Pl. 5, figs. 13, 14. 
1949 Ligonodina sp. Youngquist & Miller (partim) : 620, Pl. ror, fig. 11 only. 
1956 Ligonodina tenuis Branson & Mehl; Elias: 126, Pl. 5, figs. 4, 5. ‘ 
1957 Ligonodina sp. Rexroad : 33, Pl. 1, figs. 20, 21. 
1957 Ligonodina hamata Rexroad : 32, Pl. 1, figs. 24, 25. 
1958 Ligonodina hamata Rexroad ; Rexroad : 21, Pl. 3, figs. 9-14. 
1960 Ligonodina tulensis (Pander) Clarke 11, Pl. 2, fig. 4. 
1961 Ligonodina obunca Rexroad ; Higgins: Pl. 11, fig. 9. 
1961 Ligonodina hamata Rexroad ; Rexroad & Burton: 1154, Pl. 141, figs. 5, 6. 
1964 Ligonodina tenuis Branson & Mehl ; Rexroad & Furnish : 672, Pl. 111, fig. 4o. 
1965 Ligonodina tenuis Branson & Mehl; Rexroad & Nicoll: 22, Pl. 2, figs. 12-15. 
MATERIAL. 15 specimens : figured, X 320, X 321. 
RANGE. North Crop 3D 8-3D 22. 
DESCRIPTION. Specimens of the species, though generally fragmentary, display 
the distinctive features of the holotype. They have a characteristically strongly 
developed long, slender main denticle, which is continuously recurved, although the 
distal portion tends to be rather straight : the lateral faces are convex to flat, and 
the aboral margin tends to bear a more or less conspicuous ledge. The denticles of 
the posterior bar are relatively small in comparison with the size of the unit. 
