1853.] EGERTON PALICHTHYOI.OGIC NOTES. 281 



seen no traces of the thorns or spines on the back of the fin common 

 to all the Cestraciontidce of the Oolitic period. 



Ctenacanthus nodosus. — Lord Enniskillen has in his collection 

 an Ichthyodorulite, found also in the Dalkeith pits, differing in several 

 well-defined characters from the subject of the foregoing description. 

 It is contained in a bed of sandstone grit, alternating with shale-beds, 

 and full of vegetable remains and scales of Megalichthys. It measures 

 1 foot in length, of which 9 inches was external, and 3 invested by 

 the integuments. The width at the line of demarcation between the 

 two is 2 inches. The basal portion is comparatively short and broad, 

 and has a wide internal cavity. The obliquity of the line between it 

 and its external prolongation is more considerable than in the pre- 

 ceding species. The ornamented portion of the ray is devoid of the 

 graceful outline which characterizes many of these fossils ; it is short 

 in comparison with its breadth, and contracts rapidly in diameter. 

 The anterior profile is nearly straight for two-thirds of the entire 

 length ; it thence trends backwards with an abrupt and inelegant 

 curve. The surface is ornamented with continuous longitudinal ribs 

 or ridges, more or less constricted transversely, at irregular intervals ; 

 they agree in number with those of the preceding species. On the 

 anterior portion they deviate so slightly from the uniform character 

 prevalent in that species, that a fragment from this part of the fin 

 might be confounded with it ; but as they recede from the front they 

 are more and more deeply and frequently contracted until they become 

 broken up into distinct tubercles. This gives a knotted or monili- 

 form character to the surface of the ray very diiferent from that of 

 Ctenacanthus hybodoides. The point of the ray is rounded and 

 polished by friction, and the surface ornament for nearly an inch is 

 obliterated by the same agent. This character is also found in the 

 rays of Aster acanthus, and seems to indicate the more rocky parts of 

 the ocean as the favourite habitat of these fishes. 



On comparing the above with the genera described by Professor 

 Agassiz, they most nearly resemble the characters assigned by him to 

 the genus Ctenacanthus. This genus is common to the Carboniferous 

 Limestone of Bristol and Armagh, and has been also found in the Old 

 Red Sandstone, but has not hitherto been recorded as occurring in the 

 coal-bearing strata. The progress of our knowledge of the relations 

 of these Ichthyodorulites to the teeth with which they are associated 

 has been very slow. The arguments deduced from the numerical 

 proportions of certain forms of teeth to the rays found in the same 

 strata cannot be relied upon, neither can any sound deduction be 

 drawn from the relative sizes of the detached parts. The only con- 

 clusive evidence is the discovery of the conjunction of the teeth and 

 fins in the same specimen. By this means we now know the fin-rays 

 of Ilybodus, Acrodus, and Chimccra ; and I have recently seen in the 

 collections of Mr. Catt of Brighton and Mr. Potter of Lewes speci- 

 mens that prove the spine from the chalk named by Agassiz Sjnnax 

 major to have belonged, not to a Squaloid fish, but to a Cestracion, 



