Here again, as in the case of the Osteostraci, a relationship to the elasmobranch has been supposed, for the following reasons:—
The latest discoveries in the Silurian and Devonian deposits have brought to light strange forms such as Thelodus and Drepanaspis, of which the latter from the Devonian must, according to Traquair, be included in the Heterostraci. It possessed, as seen in Fig. [139], large plates, after the fashion of Pteraspis, and also many smaller ones.
The former, from the upper Silurian, belongs to the Cœlolepidæ, and was covered over with shagreen composed of small scutes, after the fashion of an elasmobranch. Traquair suggests that Thelodus arose from the original elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis occurred, and, with still further fusion, Pteraspis.
There are always two ways of looking at a question, and it seems to me possible and more probable to turn the matter round and to argue that the original condition of the surface-covering was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a stage on the way to the formation of an elasmobranch, and not a backward stage from the elasmobranch towards Pteraspis.
This method of looking at the problem seems to me to be more in consonance with the facts than the reverse; for, as pointed out by Jaekel, the fishes with large plates are the oldest, and in Cyathaspis, the very oldest of all, the size of the plates is most conspicuous; he considers, therefore, this preconceived view that large plates are formed by the fusion of small ones must give way to the opposite belief.
Fig. 139.—Drepanaspis. Ventral and Dorsal Aspects. (After Lankester.)
A., anus; E., lateral eyes.
So also Rohon, as quoted by Traquair, who, in his first paper accepted Lankester's view that the ridges of the pteraspidian shield were formed by the fusion of a linear arrangement of numbers of placoid scales, suggests in his second paper that these ridges may have been the most primitive condition of the dermal skeleton of the vertebrate, out of which, by differentiation, the dermal denticles (placoid scales) of the selachian, as well as their modifications in the ganoids, teleosteans, and amphibians, have arisen.
One thing is agreed upon on all sides; no sign of bone-corpuscles is to be found in this dermal covering of Pteraspis. In the deeper layers are large spaces, the so-called pulp-cavities leading into narrow canaliculi, the so-called dentine canals. The structure is looked upon as similar to that of the pulp and dentine canals of many fish-scales.