[153] According to Professor Patten's view, the close resemblance of the shields of Pteraspis to those of contemporaneous Eurypterids indicates real affinity. But the Eurypterids are related to the spiders and to Limulus. The only reason for thinking that Pteraspis is a fish at all lies in its resemblance to Cephalaspis, which is in several ways fish-like, although its head shield is much like that of Limulus. All these resemblances in Patten's view indicate real affinity. Patten considers the Pteraspids as derived from primitive arachnid or spider-like forms having a bony carapace as Limulus has. From Pteraspis he derives the other Ostracophores, and from these the sharks and other vertebrates, all of which appear later in time than the earliest Ostracophores. This view of the origin of vertebrates is recently urged with much force by Professor Patten (Amer. Nat., 1904, 1827). Most naturalists regard such resemblances in specialized structures on the outside of an animal as parallelisms due to likeness in conditions of life. The external structure in forms of really different nature is often similarly modified. Thus certain catfishes, pipefishes, sea-moths, and agonoid fishes are all provided with bony plates not unlike those of ganoid fishes, although indicative of no real affinity with them. Commonly the ancestry of vertebrates is traced through enteropneustans to soft-bodied worms which have left no trace in the rocks.
In the same connection, Professor Patten suggests that the lateral fold from which many writers have supposed that the limbs or paired fins of vertebrates is evolved is itself a resultant of the fusion of the fringing appendages on the sides of the body. Such appendages are found in the primitive mailed arachnoids and in Limulus. They are shown very plainly in Patten's restoration of Cephalaspis. About thirty of them of a bony nature and jointed to the body occur on either side between the gill opening and the vent.
[154] Called Cœlolepidæ by Pander and Traquair, but Cœlolepis is a later synonym of Thelodus.
[155] This name, inappropriate or meaningless, is older than Tremataspis.
[156] The earlier name of Pterichthys has been already used for a genus of living fishes.
[CHAPTER XXXIII]
ARTHRODIRES
The Arthrodires.—Another large group of extinct fishes mailed and helmeted is included under the general name of Arthrodira[157] (ἄρθρος, joint; δεῖρα, neck), or Arthrognathi (ἄρθρος, γνάθος, jaw), the latter term recently framed by Dr. Dean with a somewhat broader application than the former.
These fishes differ from the Ostracophores, on the one hand, in the possession of jaws and in the nature of their armored covering. On the other hand, the nature of these jaws, the lack of differentiation of the skeleton, and the uncertain character of the limbs separate them still more widely from the true fishes. Their place in the system is still unknown, but their origin seems as likely to be traceable to Ostracophores as to any other group.