It is possible that certain very faint ray-like markings noted by Professor Dean may be the basalia of paired fins. In such case Palæospondylus can have no affinity with the lampreys. Dr. Dean asserts that the presence of these, in view of the wide dissimilarity in other and important structures, is sufficient to remove Palæospondylus from its provisional position among the Cyclostomes. The postoccipital plates may represent a pectoral arch. It is, however, much more likely, as Dr. Traquair has insisted, that the supposed rays are due to the reflection of light from striations on the stone, and that the creature had no pectoral limbs.
The caudal fin, with its dichotomous rays, is essentially like the tail of a lamprey. This condition is, however, found in other groups of fishes, as among sharks and lung-fishes. It is, moreover, doubtful whether the rays are really dichotomous.
It is possible that Palæospondylus may be, as Huxley suggests, a larval Arthrodire. It is not probable that this is the case, but, on the other hand, Palæospondylus seems to be an immature form. According to Dr. Dean, it is more likely to prove a larval Coccosteus, or the young of some other Arthrodire, than a lamprey. Against this view must be urged the fact that the tail of Palæospondylus is not heterocercal, a fact verified by Dr. Traquair on all of his many specimens. It is more like the tail of a lamprey than that of Coccosteus. It is, however, certain that it cannot be placed in the same class with the living Cyclostomes, and that it is far more highly specialized than any of them. In a still later paper (1904) Dr. Dean shows that the fossil might as easily be considered a Chimæra as a lamprey, and repeats his conviction that it is a larval form of which the adult is still unrecognized.
We cannot go much farther than Dr. Dean's statement in 1896, that it belongs "among the curiously specialized offshoots of the early Chordates."
FOOTNOTES:
[157] "The name Arthrodira as given to Coccosteans, as distinguished from the Antiarcha, is not altogether a satisfactory one, since at least from the time of Pander the head of Pterichthys (Asterolepis) is known to be articulated with the armoring of the trunk in a way closely resembling that of Coccosteus. This term may, however, be retained as a convenient one for the order of Coccosteans, in which, together with other differentiating features, this structure is prominently evolved. A renewed examination of the subject has caused me to incline strongly to the belief, as above expressed, that Pterichthys and Coccosteans are not as widely separated in phylogeny as Smith Woodward, for example, has maintained. But, as far as present evidence goes, they appear to me certainly as distinct as fishes are from amphibia, or as reptiles are from birds or from mammals." (Dean.)
The name Placodermi used by McCoy in 1848 was applied to the Ostracophores as well as to the Arthrodires. Hay revives it as the name of a superorder to include the Antiarcha and the Arthrodira, the former being detached from the Ostracophores. This superorder is equivalent to the subclass Azygostei of Hay.
[158] Memoirs New York Academy of Sciences, 1901.
[159] It will be recalled that there is no ground for concluding that the "mandibular rami" possessed an endoskeletal core, and were comparable, therefore, to the somewhat mobile jaws of Elasmobranchs. On the other hand, there is the strongest evidence that they are entirely comparable to adjacent dermal plates. Histologically they are identical, and in certain cases their exposed surfaces bear the same tuberculation.