MESODON. 53 



Geology (no. 3-414) by the Rev. W. R. Andrews, representing an individual at least 

 14 cm. in length. 



The head in Mr. Gething's fossil (PI. XI I, fig. 4) is well preserved in direct 

 side view, with a rather large orbit, partly surrounded by the remains of an ossified 

 sclerotic. The cranial roof is shown only in internal impression, but the supra- 

 occipital is clearly not turned upwards behind, and the parietal bears the usual 

 posterior digitate process. The facial region exhibits the mesethmoidal plate and 

 the edge of the tooth-bearing vomer. The mandibular suspensorium is obscure 

 both in this and in the type specimen, but a fortunate fracture displays the left 

 splenial dentition from its attached face (fig. 4 a). The teeth preserved are in 

 three regular series, the large principal teeth being about twice as broad as long, 

 those of the next series also broader than long, but those of the outer series nearly 

 round. There are traces of a shallow apical pit in the crown of two principal 

 splenial teeth in the large specimen ; and some of the lateral vomerine teeth are 

 both pitted and faintly crimped. The two dentary teeth (fig. 4 a, d.),&$ usual, are 

 chisel-shaped, and the outer of the two is comparatively small. 



The maximum width of the nearly triangular preoperculura (PI. XIT, fig. 3, 

 pop.) somewhat exceeds half its depth. The deep and narrow operculum (op.) is 

 comparatively small. Both these bones are ornamented with sparse and partly 

 reticulating ridges, which radiate backwards from a point on the front margin. 

 Beneath the preoperculum are two branchiostegal rays (br.), of which the upper is 

 the larger though both are relatively small. There are distinct traces of calcified 

 gill-supports. 



The axial skeleton of the trunk closely resembles that already described in 

 M. daviesi (p. 51), but there seem to be only 11 haemal arches in the caudal 

 region in advance of the tail. The stout ribs are hollow in the fossils, and the two 

 narrow wings in the upper half of each are distinct. 



The pectoral fin, with the seven hour-glass-shaped supports in its basal lobe, is 

 conspicuous on the flank above the lower expansion of the clavicle (PI. XII, fig. 3). 

 The comparatively small pelvic fin, with about 5 slender and much bifurcated rays, 

 is also well seen in the type specimen (PI. XII, fig. 3), inserted much nearer to the 

 anal than to the pectoral tin. The dorsal and anal fins exhibit 36 and -6 supports 

 respectively in both the small specimens, and their rays are remarkably slender 

 and well-spaced, with less distal bifurcation than usual. The rays in the anal fin 

 of the larger specimen, however, are stouter and more extensively bifurcated. 

 Perhaps the first condition is a mark of immaturity. The caudal fin comprises 

 18 rays, with two or three slender fulcral rays at the origin above and below. 



The scales are as in M, daviesi (p. 51), except that the smooth denticles on the 

 dorsal and ventral ridges are coarser. Each dorsal ridge-scale bears three or four 

 denticles which increase slightly in size backwards. Each ventral ridge-scale is 

 more strongly armoured with only two or three denticles, of which the hinder is 



