ON CTENODUS. 419 



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

 scribed ; the greater portion, however, of the scale is preserved ; 

 but the border of one side is gone, as well as the posterior mar- 

 gin and part of the anterior. The sides are slightly convex, and 

 so is the anterior extremity, the angles being rounded ; the bor- 

 der (c) is wide, and distinguished by several concentric lines of 

 growth and fine minute radiating striae, 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 fragment (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 strise. 

 The bony network of the central area is fine and indistinct, with 

 a longitudinal arrangement of the meshes, as in the second spe- 

 cies ; the punctures are numerous, rather large, and longitudi- 

 nally oval. 



The last description is apparently of a mere cast of the under 

 surface; but a small portion of the scale, exhibiting the upper 

 surface, is adherent, and proves that it is minutely striated in 

 an irregular broken manner, the strise for the most part having 

 a longitudinal disposition. 



The peculiar rectangular form distinguishes these from all the 

 cycloid scales with which we are acquainted ; and they are much 

 thinner than any other of the large scales of the Coal-Measure 

 fishes. The only scale that can be compared to them in this 

 respect is that usually attributed to Rhizodus — the scale which 

 we described some time ago as belonging to Archichthys.* But 

 this scale is pretty regularly rounded, is more coarsely granu- 

 lated on the surface, and usually exhibits concentric lines of 

 growth over the whole surface ; it is also generally found split 



* Ann. Naf. Hist., Ser. 4, VoL V., p. 266. 



