In a recent paper Dr. Otto Jaekel unites Arthrodires and Ostracophores under the name Placodermi. He regards Pteraspis as a larval type, Asterolepis as one more specialized. In Coccosteus he claims to find a pelvic girdle as well as a more segmented skeleton. He regards all of these as true fishes, the Coccosteidæ as ancestral, related on the one hand to the Crossopterygians, and on the other to the Stegocephali and other ancestral Amphibians.
Suborder Cycliæ.—We may append to the Arthrodira as a possible suborder the group called Cycliæ by Dr. Gill, based on a single imperfectly known species. Few organisms discovered in recent times have excited as much interest as this minute fish-like creature, called Palæospondylus gunni, discovered in 1890 by Dr. R. H. Traquair in the flagstones of Caithness in Scotland. Many specimens have been obtained, none more than an inch and a half long. Its structure and systematic position have been discussed by Dr. R. H. Traquair, by Woodward, Gill, Gegenbaur, and recently by Dean, from whose valuable memoir on "The Devonian Lamprey" we make several quotations.
Fig. 369.—Palæospondylus gunni Traquair. Devonian. (After Traquair and Dean.)
Palæospondylus.—According to Dr. Traquair: "The Palæospondylus gunni is a very small organism, usually under one inch in length, though exceptionally large specimens occasionally measure one inch and a half.... It has a head and vertebral column, but no trace of jaws or limbs; and, strange to say, all the specimens are seen only from the ventral aspect, as is shown by the relation of the neural arches to the vertebral centra.
"The head is in most cases much eroded.... It is divided by a notch ... into two parts.... The anterior part shows a groove the edges of which are elevated, while the surface on each side shows two depressions, like fenestræ, though perhaps they are not completely perforated, and also a groove partially divided off, posteriorly and externally, a small lobe. In front there is a ring-like opening ... surrounded by small pointed cirri, four ventrally, at least five dorsally, and two long lateral ones which seem to arise inside the margin of the ring instead of from its rim like the others. The posterior part of the cranium is flattened, but the median groove is still observable. Connected with the posterior or occipital aspect of the skull are two small narrow plates which lie closely alongside the first half-dozen vertebræ.
"The bodies of the vertebræ are hollow or ring-like, and those immediately in front are separated from each other by perceptible intervals; their surfaces are marked with a few little longitudinal grooves, of which one is median. They are provided with neural arches, which are at first short and quadrate, but towards the caudal extremity lengthen out into slender neural spines, which form the dorsal expansion of a caudal fin, while shorter hæmal ones are also developed on the ventral aspect."
Dr. Traquair concludes that "there seems to be no escape from the conclusion that the little creature must be classed as a Marsipobranch." "If Palæospondylus is not a Marsipobranch, it is quite impossible to refer it to any other existing group of vertebrates."
Gill on Palæospondylus.—In 1896 Dr. Gill proposed to regard Palæospondylus provisionally as the type of a distinct order of Cyclostomes to be called Cycliæ (κύκλος, circle), from the median ring on the head, whether nostril or mouth. Dr. Gill observes:
"Assuming the correctness of Dr. Traquair's description and figures, we certainly have a remarkable combination of characters. On the one hand, if the 'median opening or rim' is indeed nasal, the animal certainly cannot be referred to the class of Selachians or of Teleostomes. On the other hand, the cranium and the segmental vertebral column indicate a more advanced stage of development of the vertebrate line than that from 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.