352 PALAEONTOLOGY OF ILLINOIS. 



4th. If it were the homologue of the embryonic half of the rostrum of 

 Pristis, it must have been placed on the side of the snout, separated from its 

 fellow, as it shows no points of contact; on the contrary, a cutting or serrated 

 edge at the summit. On this supposition, its flattened sides must have been 

 more or less horizontal, but if that had been its position, the upper and lower 

 sides would hardly have been equally arched, and the organ transversely sym- 

 metrical. 



"We are, therefore, driven by this perfect bi-lateral symmetry to suppose this 

 was not one of a pair, but that it stood alone, somewhere in the medial line, 

 either as the homologue of the sword in Xiphias, or of the rostrum in Pristis 

 (in which case it should have had an articulated base) ; or, as the homologue 

 of the dorsal spines of Chimsera, Spinax, Hybodus, Ctenacanthus, etc., or the 

 caudal spine of Trygon and the other Sting-Rays. 



There are one or two anomalous features in this fossil which require notice : 

 and first, there is no distinct line of demarcation between the exposed portion 

 and that buried in the integuments, though it is plain to see that the rough- 

 ened, knobby, basal portion was implanted in tissue, while the smooth, polished, 

 and keen-edged upper portion was as certainly exposed. In most fin spines of 

 sharks and rays, the line of the dorsal surface is very plainly marked. This 

 is not always the case, however, so that no great importance can be attached 

 to that feature. Another peculiarity of this spine is the comparative insignifi- 

 cance in size of the medullary cavity. In the great spines of Hi/bodus, Gyra- 

 canthus,Oracanthus^etG., the medullary cavity is very conspicuous, but in Edes- 

 tus it is hardly observable, and the basal extremity, which in most species of 

 ffybodus, etc., is a mere shell, is here quite solid. In some of the spines of 

 rays, however, there is scarce any medullary cavity, so that this feature need 

 not be considered incompatible with the conclusion that our fossil is a spine. 



The segmented structure of the fossil is its most marked and anomalous fea- 

 ture, but one equally so whether it be considered spine or jaw, and for which 

 no parallel suggests itself. It is undoubtedly to this structure that we must as- 

 cribe the absence of a large medullary cavity, as each segment seems to have 

 been nourished somewhat independently of its fellows. 



It is also evident that this spine was implanted in the integuments at a low 

 angle, and that an investing skin or other nutrient tissue covered fully half its 

 surface, on the lower portion reaching up to the enameled bases of the denti- 

 cles. This is the relative position of the defensive spines of rays, to which an 

 analogy is suggested by this character. 



In some plagiostomous fishes, a bone is found quite buried in the integuments 

 of the back, and which is a rudimentary representative of a posterior dorsal fin; 

 it is, therefore, not impossible that we have in the fossil before us, a higher de- 

 velopment and special modification of that organ. 



