SUPPLEMENT. 139 



head. The lower branchiostegal rays are very slender. The parts of the trunk 

 preserved seem to be almost in their natural position, but some of the abdominal 

 vertebra? are displaced above the fossil, while the upper lobe of the caudal fin is 

 torn away and displaced below. The axial skeleton is typically that of Thrissops, 

 and the middle haemal spine at the base of the caudal fin is much- expanded. The 

 stoutness and length of the anterior supports of the anal fin show that this must 

 have been very deep and acuminate in front. 



Horizon and Locality. — Middle Purbeck Beds : Swanage, Dorsetshire. 



SUPPLEMENT. 



Hybodus basanus, Egerton (p. 5). Plate XXVI, fig. 3. 



Mr. Reginald W. Hooley, F.G.S., has obtained from the top of the Weald Clay 

 in the typical locality, Atherfield, Isle of Wight, the well-preserved small head 

 partly shown in PI. XXVI, fig. 3. In the front part of the jaws it displays some 

 of the characteristic teeth ; and at the limits of the gape of the mouth there are 

 remains of the labial cartilages, as already described (p. 7, PI. II, figs. 1 a, 1 b). 

 Even allowing for some vertical crushing, however, the skull clearly differs from 

 the well-preserved specimen from Pevensey (PI. II, fig. 1) in being relatively 

 shorter and broader, with less widened postorbital processes. As seen from above 

 (PL XXVI, fig. 3), the supraorbital flanges are well developed, and the interorbital 

 width exceeds half the total length of the cranial roof. The posterior depression 

 (p.f.) is less elongated than in the best-preserved Pevensey specimen. The small 

 rostral prominence is evidently broken away by accident. The occiput (PI. XXVI, 

 fig. 3 ft) is especially well shown, only abraded at its median vertical crest, which 

 is still best seen in the skull already mentioned on p. (3. The occipital face is more 

 than half as deep as wide, and its most conspicuous feature is the large excavation 

 for the notochord (».) in the basioccipital region, resembling that in Notidanus 

 (Hexanchus) . l The foramen magnum (f.m.), seen above this excavation, is com- 

 paratively small. A pair of deep pits (v.) in the cartilage flanking the foramen 

 magnum would probably be pierced by the foramina of the vagus nerves. 



Hybodus sulcatus, Agassiz. 



The two fragments of a dorsal fin-spine named Hybodus sulcatus by Agassis 

 (Poiss. Foss., vol. iii, 1837, p. 44, pi. x b, figs. 15, 16), though originally stated by 



1 C. Gegenbaur, Unteisuchungen zuv vergleichenden Anatomie der Wirbelthiere. — III. Das 

 Kopfskelet der Selachier (1872), p. 120, pi. iv, fig. 2, pi. xv, fig. 2. 



