80 FOSSIL FISHES OF THE ENGLISH CHALK. 



longitudinal in direction and most conspicuous near the hinder end and oral border. 

 The bone seems to have been traversed by a slime-canal just below the level of the 

 anterior ascending process ; while its upper part is overlapped by two large 

 supramaxillse. The latter plates (PI. XVII, figs. 1, 2, smx. ; PI. XVIII, fig. 1, 

 smx.) are also finely ornamented with vermiculating rugae. The posterior and 

 larger of these plates (smx. 2) is irregularly triangular in shape, with the apex 

 behind, and its antero-superior angle produced forwards into a slender point (PI. 

 XVII, fig. 1, smx. 2 ; also seen in B. M. no. P. 7652). It is considerably longer 

 than deep, and the most conspicuous feature of its external ornament is a wavy, 

 longitudinal groove extending along its middle. The anterior supramaxilla (smx. 1) 

 is rounded behind and tapers in front, where its smoothness seems to indicate overlap 

 by the large foremost plate of the circumorbital ring. It is twice as long as deep, 

 its length equalling that of the posterior supramaxilla. The mandible is remark- 

 ably short and deep, the depth in the coronoid region equalling nearly half of its 

 total length. The articulo-angular bone is comparatively small, extending only 

 into the lower half of the coronoid region. It is quite smooth except at the 

 thickened lower edge, which is slightly rugose. The greater part of the dentary 

 bone is exposed in the type specimen figured by Agassiz, and portions of it are also 

 seen in the originals of PL XVII, figs. 1, 2 (d.) ; PI. XVIII, fig. 1 (d.). It is 

 truncated at the symphysis and rapidly rises into the great coronoid process. Its 

 outer face is smooth except at the lower border, where it is traversed by a deep 

 longitudinal groove for a slime-canal, and the upper edge of this is sometimes 

 slightly rugose. The teeth indicated by Agassiz can hardly be seen in the type 

 specimen; but such minute conical teeth were clearly present (B. M. no. P. 5418). 



In the hyoid arch the epihyal (PI. XVIII, fig. 2, eph.) is slightly longer than 

 deep, while the ceratohyal (fig. 3, ch.) is comparatively long and slender, with its 

 \\\\\)W cuds united by ;i supplementary plate of bone. The urohyal (PI. XVIII, 

 fig. 4) is a relatively large thin bony lamina, quadrangular in shape and not much 

 longer than deep. As shown by ;i fragment represented in PI. XVII, fig. 4, the 

 branchial arches bear a close series of long and slender gill-rakers. 



Tlic cheek is covered partly by circumorbital plates, partly by scales. Most of 

 the circumorbital plates (PI. XVII, fig. \,co.) are small and form a very narrow rim 

 behind and below the orbit; but the foremost element of the series, which may be 

 termed antorbital (ao.), is relatively large and expanded. These plates are 

 traversed by ;i deep groove for the slime-canal, and their prominent orbital rim is 

 ornamented by coarse ruga' or tubercles. The large antorbital, well seen in 

 PL XVII, fig. 1 (ao.), is deepest at the anterior end, where its depth equals about 

 half its extreme length. Its coarsely rugose, everted orbital rim is especially large, 

 overhanging ac extensive hollow for the slime-canal; and below this the flat 

 lamina of tin- bone is ornamented with fine tubercles and ridges, which are directed 

 chieflv at right angles to the lower edge. The scales behind and below the 



