HYPSODON ETC. FROM BRITISH CRETACEOUS STRATA. 515 



a smooth outline, indicative of a cartilaginous articulation with the 

 hypo- and epihyals, the anterior being convex and the posterior 

 concave. The lower margin has a groove, which is widest and 

 deepest at the narrow part of the bone, and is gradually lost before 

 reaching the hinder end ; the posterior three fourths of an inch of 

 this margin being a thin, sharp edge. The upper margin is a thin 

 edge from end to end. 



Immediately in front of the ceratohyals there are portions of 

 three or four bones which, from their position, seem to be the double 

 hypohyals (basihyals of some authors); but this is uncertain, as 

 they are broken, and their true form cannot be made out. There 

 are no traces of any branchiostegal rays. 



The bone marked bh in figure 5, which is below the hypohyals, 

 is compressed anteriorly, but becomes stouter and rod-like poste- 

 riorly. Just above these letters the upper margin is expanded 

 laterally into two short wings, having a deep groove between them. 

 The hinder edge of this bone seems to be naturally flattened on 

 each side, and is wedged in between two fragments of bone (cl, cl') 

 which appear to be the anterior extremities of the clavicles. The 

 bone bh is, in all probability, the basihyal (glossohyal), which has 

 been thrown out of place. If, however, the two fragments of bone 

 (cl) are really the ends of the clavicles (and there appears to be no 

 good reason for doubting this), then the bone bh occupies precisely 

 the position of an interclavicle ; and it is not a little remarkable 

 that this peculiarity should occur in a group of fishes having other 

 saurian characters. 



Lying between the two ceratohyals, and, indeed, hidden by them 

 until they were separated in the process of cleaning the specimen, 

 there is an extremely thin bone, almost equalling them in size 

 (PI. XXII. fig. 8).* Its hinder end is broken off; but otherwise 

 it is perfect. The form and position of this bone, lying, as it 

 does, in the reentering angle of the hyoid arch, show clearly 

 that it corresponds to that which Dr. Parker has called the 

 " basibranchiostegal." One feature of this bone which deserves 

 notice is, that its upper portion is covered with thin scale-like plates, 

 which at first might be thought to be the scales. A closer exami- 

 nation, however, shows that such is not the case, but that they are 

 dental armatures. Near the anterior and upper part of the bone 

 there are a few plates (fig. 8, a) marked by ridges and grooves which 

 are directed upwards and forwards ; and when these are examined 

 with a microscope the surfaces appear smooth and dense in structure. 

 Passing backwards these plates are found gradually to lose their 

 ridges, and about the middle of the bone have their surfaces marked 

 by irregular granulations, which are visible with a pocket-lens. 

 Towards the hinder part of the bone there are a number of smaller 

 plates with radiating ridges (fig. 8, b) ; and when magnified, their 

 entire surfaces are seen to be covered with small pits ; these are the 

 bases of minute teeth, some of which are still in place, and others 

 may be seen lying around. At the anterior part of the bone, and 

 * Fig. 8 has by some mishap been drawn upside down. 



