f) \YEALDEN AND PURBECK FOSSIL FISHES. 



mandible discovered by Capt. L. L. Boscawen Ibbetson at the top of tlie Wealden 

 near Atherfield, Isle of Wight, and is now in the Mnsenm of Practical Geology. 

 Since its description by Egerton (loc. cif., 1845), it has been cleaned from tlie 

 matrix, and new drawings of the specimen from the right side and from below are 

 given in PI. T, figs. 1, 1 (i. All the cartilages are distorted by crushing, and the 

 waterworn teeth are less distinct than indicated in Egerton's original figure, 

 where each principal cusp appears too wide and smooth. 



All the other known specimens are similar heads and fragments of the trunk 

 })icked up on the beach of Pevensey Bay, Sussex, where a large collection, now in 

 the British Museum, Avas made by Mr. S. H. Beckles. From these fossils the 

 principal characters of the species and several interesting anatomical features can 

 be determined. 



The cartilages agree with those of modern sharks in Ijeing only superficially 

 calcified in the usual small polygonal tesserjs. They are therefore often distorted, 

 not merely by crushing during fossilisat ion, Init also by contraction before burial 

 in the sediment. Under such circumstances their state of preservation is remark- 

 al)]e. In most cases the hollow left by the decay of the internal uncalcified 

 cartilage is filled with ordinary matrix; but sometimes (as in the original of PI. II, 

 fig. 1) it still remains partly vacant. 



The cranium as shown in the type specimen (PI. I, fig. 1) is rather short and 

 wide, with a relatively large orbit {orb.), short postorl^ital and rostral regions, and 

 a large anterior fontanelle {'(■■/). Its special features, however, are better seen in 

 other specimens, particularly in the unirpie skull represented in Pl.II, figs. 1, 1 (i, 

 1 h. This lacks only the occipital region, which is preserved in another specimen 

 in the Enniskillen Collection (B. M. no. P. 8172 c), and is seen to slope backwards 

 and downwards, while it is raised in the middle into a sharp vertical ridge extend- 

 ing from the occipital border to the foramen magnum. The cranial roof throughout 

 its length (PI. II, fig. 1) is gently convex from side to side, is produced downwards 

 into a large postorbital process (fig. 1 a), and extends above the orbit into a thin 

 supraorbital flange, which merges in front into the depressed and only slightly 

 expanded region of the nasal capsule. In the middle of the roof of the post- 

 orbital region the posterior fontanelle {'p-f.) is elongate-oval in shape. In front 

 of and between the nasal capsules, the large anterior fontanelle ((/./. ) is much 

 broader than deep and is directed forwards ; while the flat base of the 

 mesethmoidal region soon terminates in a very short but well-marked rostral 

 prominence (r.). 



The mandibular suspensorium is inclined backwards, so that, since the jaws 

 extend forwards as far as the end of the snout, they are longer than the cranium. 

 As shown by the type specimen (PI. I, fig. 1) they are also relatively large and 

 massive, with lal)ial cartilages at the angle of the mouth. The hyomandibular 

 (PI. II, figs. 1, 1 (I, hm.) is a comparatively slender cartilage, laterally compressed 



