L50 FOSSIL FISHES OF THE ENGLISH CHALK. 



pressed to a pair of sharp edges, which are not serrated. Their enamelled surface 

 is vertically wrinkled or striated, and often cracks along the fine lines thus 

 formed. In transverse section, when viewed with a microscope (fig. 8) they are 

 shown to be composite, consisting of crowded tubules of irregular size and shape. 

 From the central canal of each tubule there radiate numerous branching canaliculi. 

 The successional tooth appears directly below the functional tooth, and produces 

 a cavity by absorption in the side of the base of the latter (fig. 7, s.). 



Of the axial skeleton of the trunk nothing is definitely known, but there 

 cannot be much doubt that a peculiar form of hypural bone found in the Chalk, 

 (iault, and Cambridge Greensand, is rightly referred to Protosphyreena (W. Davies, 

 he. cit., 1878, p. 256). The specimens from the Chalk must belong chiefly to 

 P. ferox. One such hypural has actually been found partly overlapped by the 

 rays of the lower lobe of the fin, and is shown in PI. XXXII, fig. 2. An isolated 

 specimen from the Cambridge Greensand is represented for comparison in fig. 3. 

 Though laterally compressed, the bone is slightly tumid at the sides, and it is 

 marked by feeble, nearly vertical grooves to accommodate the deeply-overlapping 

 caudal fin-rays (v.). It is usually more than twice as deep as wide, with rounded 

 angles ; and the middle of the posterior border is compressed to a sharp edge. 

 It obviously represents a single, enormously-expanded haemal arch, and its base 

 appears as a relatively small process (p.) above the middle point of its anterior 

 border. Above this process the anterior border is excavated by a longitudinal 

 groove. 



The fins of Protosphyrsena hitherto discovered in the English Chalk are all 

 fragmentary and isolated, but most of them agree more or less closely with the 

 pectoral fins of P. perniciosa from the Chalk of Kansas (Text-fig. 43, p. 146) and 

 probably represent the pectorals of P. ferox. Their undivided rays are firmly 

 pressed together and successively terminate at the sharp front border, which is 

 almost straight near the base, but soon becomes wavy further down, and eventually 

 rises into a row of triangular teeth (PI. XXXII, fig. 4). For the most part the 

 rays are smooth, but at the front border they are finely rugose, with the rugae 

 directed transversely. At the straight base of insertion of the fin, the right and 

 left halves of the rays diverge to clasp the baseosts, of which there are eight. 

 The foremost baseost (PI. XXXII, figs. 5, 5 a, l ; also seen in Brit. Mus. no. P. 

 4507 a) is short and stout, deeply pressed between the rays, inclined towards the 

 outer or upper side, and hollowed at its summit into a regular circular cup for 

 articulation with a rounded prominence on the scapula (Text-fig. If). The 

 second baseost (described as a pair in the specimens from the American Chalk) 

 is a quadrangular plate (figs. 5, 5 a, n) nearly twice as wide as deep, fixed at 

 right angles to the plane of the fin, attenuated in the middle, and unequally 

 thickened at its extremities, where it articulates with the pectoral arch in two 

 separate flattened facettes, of which the inner or lower is the larger. The 



