MESODON. 51 



the transversely elongated principal splenial teeth are smooth and rounded, without 

 any apical pit ; the teeth of the first outer flanking series are also smooth, broader 

 than long, and about half as broad as the principal teeth ; while (as seen in the 

 counterpart of the same specimen) the teeth of the second flanking series are 

 somewhat smaller, but still broader than long. One tooth of the fir^t flanking 

 series exhibits an apical pit. The dentary bone (PI. XII, fig. 2, d.), of the usual 

 Pycnodont shape, bears two chisel-shaped teeth, of which the inner is the larger. 



The triangular preoperculum (PI. XII, figs. 1, 2, pop.) does not extend so far 

 upwards as the relatively small, deep, and narrow operculum (pp.) ; and when well 

 preserved the radiating reticulated ornament of its outer face is very conspicuous 

 (fig. 2, pop.). Traces of calcified gill-supports are seen beneath it in the Dorset 

 Museum specimen. 



The vacant space for the notochord forms a nearly straight band somewhat 

 above the middle of the trunk, and the vertebral arches above and below it exhibit 

 a small basal triangular expansion. In the type specimen (PL XII, fig. 1) 33 or 

 34 neural arches can be counted, the foremost being the stoutest and most Avidely 

 spaced, while the hindmost three or four are comparatively diminutive and related 

 to the support of the caudal fin. Most of these arches bear traces of the anterior 

 laminar expansion (seen especially in the counterpart of the type), and about 18 may 

 be reckoned as belonging to the abdominal region. The ribs are much expanded in 

 their proximal portion and stout, but do not reach the ventral border of the trunk. 

 There are about 13 hasmal arches in advance of the tail in the caudal region, nearly 

 symmetrical with the opposed neurals ; and at least 7 comparatively short hsemals 

 are crowded together within the base of the caudal fin. 



Of the paired fins, only traces of the pectoral are seen in the type specimen. 

 Seven stout hour-glass-shaped supports are preserved in the basal lobe, and the 

 expanse of the fin must have been large, with numerous closely articulated rays. 

 Thirty-nine supports can be counted in the dorsal fin, and about 30 supports in the 

 anal fin ; and the dorsal fin is sufficiently well preserved to show that the foremost 

 three or four rays' gradually increase in length to the longest. In both these fins 

 the articulations of the rays are close, and the unarticulated base is very short. 

 In the caudal fin there are slightly more than 20 rays, of which those at the upper 

 and lower borders are much crowded, and may be described as partly fulcral. 



Remains of the scales cover the whole of the anterior half of the trunk to the 

 origin of the median fins, arranged in 14 transverse series, with traces of the upper 

 end of two more series just in front of the dorsal fin. For the greater part of their 

 length the series of scales are represented in the fossils merely by the thickened 

 inner rib which bears the peg-and-socket articulation. It is only in the lower part 

 of the abdominal region that they are complete. Here they are thick and smooth, 

 with an especially wide inner rib and large peg-and-socket articulation. The 

 ventral ridge-scales are saddle-shaped and (as shown in the Dorset Museum 



