G-IGANTIC BIRD FROM LOWER EOCENE BEDS. 145 



II. — Description op the Specimens. 



Tilio-tarsi. The first specimen to be described is that represented by figures 7 to 11, 

 PI. XXIX. ; it is the lower 4 or 5 inches of a left tibio-tarsus, the upper portions of 

 which were not recovered, although the recent fracture shows that more of it was 

 present. So much of the bone as we have is very perfectly preserved, and allows every 

 detail of its structure to be studied, although in some parts it is covered with small 

 rounded bosses of iron pyrites. 



The most striking character of this bone, when seen from the front (PI. XXIX. 

 fig. 7), is the osseous supratendinal bridge [s.t.i), which at once proves it to be of avian 

 origin. The groove (t.c) for the tendon of the common flexor muscle of the digits, 

 which passes under the osseous bridge, is deep, and situated towards the inner side of 

 the bone. The inner margin of this groove is well defined ; but on the outer side there 

 is no distinct boundary. The upper margin of the bridge is thin, and curves from the 

 inner side outwards and upwards ; the lower margin has a thickened upturned lip, and 

 is directed outwards and downwards ; so that the bridge itself is broadest towards the 

 outer side *. The canal passing beneath the osseous bridge has an almost triangular 

 lower opening, which, in consequence of the projection of the bridge at its lower margin, 

 looks downwards and inwards. Immediately above the bridge the inner margin of the 

 groove presents a thickened and rugose surface for the attachment of the oblique liga- 

 mentous bridge, the lower and outer end of which was fixed to the oval roughened 

 space (La) seen on the outer part of the osseous bridge. On the outer side of the oval 

 space (La) there is a depression or groove (t.g) for the tendon of the peroneus muscle, 

 and the roughened surfaces on each side of this groove doubtless indicate the points of 

 attachment of the second ligamentous arch. Below the osseous bridge, and just above 

 the margin of the intercondylar trochlear surface, there is a deep depression (ic.d) ; 

 above this is a roughened area, which extends upwards to the outer side of the bridge. 



The articular condyles are large, the inner one (i.c) being larger then the outer (o.c), 

 and projecting inwards beyond the general contour of the bone (PI. XXIX. fig. 7), 

 while the outer condyle is rather within the line of the outer side of the bone (compare 

 fig. 2, PI. XXVIII.). The inferior trochlear surface is somewhat rough and irregular : 

 in a front view (PI. XXIX. fig. 7) it is seen to be concave from side to side. Or, more 

 correctly, the lower surface of the outer condyle forms an angle with that of the inner 

 condyle, the point of junction (b) being situated at about the outer third of the bone, 

 and consequently the lower surface of the inner condyle has about twice the width of 

 that of the outer condyle. This angle becomes less marked towards the hinder part of 

 the bone. When the trochlear surface is viewed from below (PI. XXIX. fig. 11) the 

 greater antero-posterior extent of the inner condyle (i.c) is most obvious. On the hinder 

 part (pt.s) a definite and regular concavity connects the inner and outer parts of the 

 * These characters are not well represented in this figure, but are better shown in figure 2. 



z2 



