The Class Ostracophori 569 



Nature of the Ostracophores. The Ostracophores are found 

 in the Ordovician or Lower Silurian rocks, in the Upper Silu- 

 rian, and in the Devonian. After the latter period they dis- 

 appear. The species are very numerous and varied. Their 

 real affinities have been much disputed. Zittel leaves them 

 with the Ganoids, where Agassiz early placed them, but they 

 show little homology in structure with the true Ganoids. Some 

 have regarded them as aberrant Teleosts, possibly as freakish 

 catfishes. Cope saw in them a huge mailed group of archaic 

 Tunicates, while Patten has soberly and with considerable 

 plausibility urged their affinity* to the group of spiders, es- 

 pecially to the horseshoe-crabs (Limulus) and their palaeozoic 

 ancestors, the Eurypterids and Merostomata. 



The best guess as to the affinities of the Ostracophores is 

 perhaps that given by Dr. Ramsey H. Traquair (" Fossil Fishes 

 of the Silurian Rocks of the South of Scotland," 1899). Tra- 

 quair regards them as highly aberrant sharks, or, more exactly, 

 as being derived, like the Chimaeras, from a primitive Elas- 



* 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 Ostra- 

 cophores. 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 enterop- 

 neustans 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 verte- 

 brates 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 restora- 

 tion 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. 



