TOMOGNATHUS. 141 



single and narrow ; but it has not been clearly observed. The mandible is much 

 deepened in its hinder half, and the articulo-angular bone (figs. 5, (3, ag.) is 

 relatively small and short, bearing the articular facette at a low level. The long 

 and rounded coronoid elevation is formed entirely by the dentary (figs. 4 — 7, d.), 

 which contracts into a stout narrow bar in its short tooth-bearing portion. This 

 narrow bar, which is somewhat widened in a horizontal plane, curves inwards to 

 meet its fellow of the opposite side at the symphysis. Its outer face is coarsely 

 rugose and pitted, with one conspicuous longitudinal series of large pits, which is 

 continued along the lower edge of the mandible behind, evidently marking the 

 course of a well-developed slime-apparatus. The outer face of the large coronoid 

 region is smooth. Three of the anterior mandibular teeth are much enlarged, like 

 those of the premaxilla, but the tooth immediately at the symphysis is smaller, 

 while those behind the large teeth rapidly diminish in size backwards. On the 

 inner side of the principal mandibular teeth there is another regular close series of 

 comparatively minute conical teeth, beginning at the symphysis and extending 

 backwards beyond the outer series for some distance up the slope of the coronoid 

 region (fig. 5). These small teeth are well shown in several specimens, but it is 

 not clear whether they are borne by a separate splenial element. One imperfect 

 mandible in the Bowerbank Collection (B. M. no. 39051) rather favours the latter 

 supposition. All the teeth are hollow cones, not in sockets but directly fused with 

 the supporting bone. They have smooth enamel, which does not extend to the 

 base ; and there are usually faint vertical flutings on the larger crowns (fig. 4 a). 



The opercular apparatus is small and delicate, though complete and succeeded 

 beneath by a few branch iostegal rays (at least six or seven in B. M. no. 4970G). 

 The preoperculum (fig. 6, pop.) is narrow and arcuate, not produced into a lower 

 limb ; its outer face is smooth and concave. The operculum (op.) and suboperculum 

 (sop.) are also nearly smooth, though sometimes with a trace of coarse rugosity and 

 tuberculatum. No remains of a gular plate have been observed. 



The trunk is almost unknown, but the fragmentary specimen shown in 

 PI. XXIX, fig. 13, suggests that the fish was long-bodied rather than fusiform. 

 The vertebral centra have not been seen, and it is difficult to distinguish parts of 

 vertebral arches from parts of fin-rays in fossils such as that just mentioned. The 

 endoskeleton must have been feebly ossified, as is indicated by remains of some of 

 the expanded haemal elements at the base of the caudal fin (c). The clavicle 

 (fig. 12, el.) tapers below and forwards, but is much expanded in its ascending- 

 portion, which appears to be short and sharply truncated at its upper end. The 

 pectoral fin, of which the base is well shown in fig. 12, is relatively large and 

 expanded. It comprises at least 19 rays, of which the foremost is the stoutest 

 and is inserted by an expanded base which rises above the level of the other rays. 

 The pelvic fins are unknown, but two feebly ossified elements shown in the middle 

 of fig. 1 3, ph., are suggestive of their bony supports. A few portions of fin-rays 



