258 Messrs. Hancock and Attliey on Ctenodus. 



are thin and delicate, and apparently represent lliree species, 

 though the distinguishing characters are slight. 



The first (PL XVIII. fig. 1), the largest and most perfect 

 specimen, measures two and a half inches long, and upwards 

 of two inches wide. The sides are parallel ; the anterior ex- 

 tremity {a) is a little arched outwards, and the posterior or 

 exposed extremity ih) is rounded ; the angles are rounded off j 

 the central area (e), under an ordinary hand-lens, appears 

 quite smooth, and is bordered by a rather narrow margin (c), 

 having several concentric undulations or lines of growth, and 

 marked with minute radiating stria3 ; no growth-lines are 

 visible within the marginal border. On examination with the 

 inch object-glass, the central area is found to be finely reticu- 

 lated with slightly elevated bony fibres, the meshes being sunk, 

 so that the surface is minutely and regularly punctate. This 

 is undoubtedly the underside of the scale ; the upper surface 

 is revealed on fragments, and, at a rupture {d) near the centre 

 of the rounded exposed extremity, is minutely granular. Of 

 course, in the latter case, it is only the cast of the upper sur- 

 face that is seen ; and at this point it is evident that the 

 granules are enlarged and become arranged so as to form im- 

 perfect and very irregular vermicular grooves. 



The second species (fig. 2) is less perfect than that just 

 described ; the greater portion, however, of the scale is pre- 

 served ; but the border of one side is gone, as well as the pos- 

 terior margin and part of the anterior. The sides are slightly 

 convex, and so is the anterior extremity, the angles being 

 rounded ; the border (c) is wide, and distinguished by several 

 concentric lines of growth and fine minute radiating striaa, as 

 in the first species. The central area [d) is likewise similar ; 

 but the minute surface-structure is finer, and the bony net- 

 work has the meshes drawn out in the long axis of the scale ; 

 the punctures, too, are not so large and distinct. This frag- 

 ment (for fragment it is) measures two inches long, and one 

 inch and one eighth wide. 



The third species, which has lost the greater portion of the 

 rounded posterior extremity, and is in other respects imperfect, 

 is upwards of an inch and three fourths long ; it seems to 

 have been more nearly square than either of the other two 

 forms, and is characterized by a very narrow border, which 

 shows only one or two concentric lines of growth and minute 

 radiating stride. The bony network of the central area is fine 

 and indistinct, with a longitudinal arrangement of the meshes, 

 as in the second species ; the punctures are numerous, rather 

 large, and longitudinally oval. 



The last description is apparently of a mere cast of the 



