Gill, 1896.—"The group to which Palæospondylus belongs may be defined as Monorrhines with a continuous (?) cranium, a median nasal (?) ring, and a segmented vertebral column." "The cranium and segmented vertebral column indicate a more advanced stage of development of the vertebrate line than that from which the living Marsipobranchs must have originated. We may, therefore, with propriety isolate it as the representative not only of a peculiar family (Palæospondylidæ), but of an order or even subclass (Cycliæ) of vertebrates which may provisionally (and only provisionally) be retained in the class of Marsipobranchs."
Dean, 1896.—"Place it with the Ostracoderms among the curiously specialized offshoots of the early Chordates, but this position would be at the best unsatisfactory."
Dean, 1898.—"Palæospondylus should not be given a place—even a provisional one—among the Marsipobranchs." To be accepted "as the representative of the new subclass (or class) Cycliæ constituted for it by Professor Gill."
Parker & Haswell, 1897.—"There is some reason to regard that Palæospondylus is referable to the Cyclostomes." "A distinctly higher type than recent forms."
Gegenbaur, 1898.—"Discovery of Palæospondylus one of the highest importance. If this organism stands in no way near the Cyclostomes, the tentacles lose their higher importance, since they also occur in other groups." "Through Palæospondylus came also the attempt (Pollard) to deduce the presence of the tentacular condition in the higher forms." (Mem.—In this Gegenbaur has not consulted the literature accurately. At the time of founding his "Cirrhostomal Theory" Pollard was unaware of the discovery of Palæospondylus). "Ich muss sagen, das die positive Behauptung der einen wie der anderen Deutung mir sehr unsicher scheint, da auch an den übrigen Resten des Kopfskelets keine bestimmten Uebereinstimmungen mit anderen Organismen erweisbar sind. Es ist daher auch nicht zu vermuthen, dass sogar an Beziehung zu Froschlarven gedacht ward. Unter diesen Umständen möchte ich jene im Verhältniss zum Kopfe wie zum gesammten Körper bedeutende, von Cirren umstellte Eingangsöffnung als nicht einer Nase, sondern einem Munde oder beiden zugleich angehörig betrachten. Zu einem dem Cyclostomenriechorgan vergleichbaren Verhalten fehlen alle Bedingungen."
Relationships of Palæospondylus.—The arguments for and against the supposition that Palæospondylus is a Cyclostome may be here summed up after Professor Dean.
The vertebral column agrees with that of the lamprey in having the notochord in part persistent. On the other hand, the vertebræ have continuous centra, showing definite processes. Those of the different regions are differentiated. These conditions are quite unlike those seen in the lamprey.
The cranium is massive, over twice as large proportionally as that of the lamprey. In the latter type the cranium forms but a small portion of the bulk of the head; in Palæospondylus, on the other hand, the cranium bears every sign of having filled the contour of the head. Moreover, if the region adjacent to the structure is admitted to be that of the eye, and few, I believe, will doubt it, then the brain-cavity must, by many analogies, have been much larger than that of a Marsipobranch. Also the auditory capsules must have been of extraordinary size. In short, there is very little about the cranium to suggest the structures of Cyclostomes.
The "oral cirri" suggest somewhat the barbels of the nose and mouth of a hagfish. They, however, resemble even as much in arrangement and greater number the buccal cirri of Amphioxus. On the other hand, similar mouth-surrounding tentacles are evolved independently in many groups of fishes, siluroids, sharks, forms like Pogonias, Hemitripterus. A possibility further exists that the "cirri" may turn out to be remnants of cranial or facial structures of an entirely different nature. In fact the very uncertain preservation of these parts renders their evidence of little definite value. In but one specimen, as far as I am aware, is there any evidence of the presence of ventral cirri.
The jaw parts in Palæospondylus are unknown. It is possible that the ventral rim of the "nasal ring" may prove to be the remains of the Meckelian cartilage (the cartilaginous core of the lower jaw).