62 WEALDEN AND PURBECK FOSSIL FISHES. 



crenulated (Brit.Mus. no. P. 1627 b). They are all closely arranged, and are often 

 much worn by mastication. The vomerine dentition is only incompletely seen in 

 the skeletons, but there can be little doubt that the large isolated example shown 

 •enlarged in PI. XIV, fig. 5, belongs to Microdon radiatus. Although this specimen 

 is much worn it still retains traces of the apical pit in some teeth, and the oral 

 surface is gently convex from side to side. The principal median teeth are 

 slightly more than twice as wide as long, and the alternating inner paired teeth 

 are relatively large. The outer paired teeth are smaller and very irregular both 

 in size and shape. The splenial dentition is well displayed in two skeletons in the 

 British Museum (PI. XIV, figs. 7, 8), both much worn, but retaining traces of the 

 apical pits in the outer or lateral teeth. The principal teeth are at least as wide 

 .as the two outer series together, and the inner series of small teeth is always 

 imperfectly developed. The teeth of the outermost series are wider than long, 

 and much larger than those of the next series. The microscopical structure of the 

 tritoral teeth has already been described by Owen ('Odontography,' 1840, p. Jo, 

 pi. xliii, fig. 1). Each tooth has a large pulp-cavity from which numerous very 

 fine tubuli radiate into the ordinary dentine as far as the thick superficial layer of 

 ganodentine. 



The hyoid arch is comparatively small, and does not bear any branchiostegal 

 rays. The epihyal (PI. XIV, fig. 9, eph.) is about half as long as the ceratohyal 

 ich), which is deepened behind and much constricted in front. 



The opercular apparatus comprises solely a large preoperculum and a smaller 

 operculum. The preoperculum, usually crushed as shown in PI. XIV, fig. 1, is 

 irregularly triangular in shape with a truncated apex, the maximum width of its 

 base considerably exceeding half its depth. It is more or less firmly united to the 

 hyomandibular, which the truncation of its apex leaves exposed (or possibly 

 covered by a separate plate) at its upper end. Its outer face is ornamented by 

 reticulating ridges which radiate from the middle of its anterior border; and the 

 usual slime-canal, which traverses the anterior half of the bone, is marked in its 

 upper portion by a slight groove, in its lower portion by a few irregularly spaced 

 pores. The operculum (PI. XIV, fig. 10, op.) is also approximately triangular in 

 shape, but widest at its upper end and from three to four times as deep as wide. 

 Its upper limit corresponds with that of the hyomandibular, and the ornamental 

 reticulations on the outer face radiate from its point of suspension. When the 

 opercular bones are removed, traces of calcified gill-supports are sometimes seen. 



The vacant space for the notochord runs in a slightly sinuous curve somewhat 

 above the middle of the trunk, and the bounding arches must have been more or 

 less firmly united, as shown by their usually undisturbed relations in the fossils. 

 Their triangular proximal expansions are much too small to encircle the notochord, 

 but their characteristic anterior laminar appendages are sufficiently wide to fill at 

 least the proximal half of the spaces between them. A few of the anterior neural 



