342 THE OSTRACODERMS AND THE MARINE ARACHNIDS. 



No doubt this was due, in part, to the remarkable development of their der- 

 mal skeleton, because, for a long time, it had been very generally assumed that an 

 animal with a continuous dermal armor could not be a primitive vertebrate, for 

 in the elasmobranchs, which were supposed to be the most primitive, the body 

 was covered with minute, isolated, dermal ossicles. It was also implicitly be- 

 lieved at that time, as it is to-day, with a conviction born of constant repetition, 

 that the immediate ancestors of the vertebrates were animals like either Amphi- 

 oxus, Balanoglossus, the tunicates, or the annelids, which had no dermal skeleton 

 whatever. 



The author (Patten, 1889) was the first one to claim for the ostracoderms an 

 important place in the phylogeny of the vertebrates, basing this claim in part 

 on the very characters which were regarded by others as evidence of their high 

 degree of specialization. 



At that time our knowledge of the ostracoderms was very imperfect and the 

 anatomical foundation for any inference in regard to them was exceedingly in- 

 secure. Since then, our knowledge has greatly increased and we now possess in 

 Bothriolepis, one of the higher representatives of the group, an unprecedented 

 wealth of material, which in spite of its great age is in an ideal state of preserva- 

 tion. Indeed, I know of no other extinct animal that has been so abundantly and 

 perfectly preserved in its original form, attitudes, and surroundings. 



This new material for the first time enables us to identify with certainty the 

 neural and haemal surface of an ostracoderm; it furnishes us the first precise in- 

 formation concerning the nature and location of the sense organs, the jaws, mouth, 

 gills, and other viscera, and as to their mode of life; and for the first time it affords 

 a secure basis of fact for the interpretation of other representatives of the group 

 that are not so well preserved. In the light of this evidence, we may now confidently 

 affirm that the ostracoderms belong neither to the arthropods nor to the vertebrates, 

 but constitute a new class standing midway between them, the ancestors of the 

 one and the descendants of the other, the long sought missing link between the 

 vertebrates and the invertebrates. 



From the geological record we may conclude that the true vertebrates arose 

 from the ostracoderms not later than the Silurian; the ostracoderms from the 

 marine arachnids not later than the Ordovician; while the marine arachnids had 

 their origin in the immense, unfathomable periods during, or preceding, the 

 Proterozoic. 



Historical Review. 



Before proceeding to a fuller description of the ostracoderms, we may to 

 advantage review the earlier literature on their structure and systematic posi- 

 tion, for it brings out the most striking features of such important genera as 

 Pterichthys, Pteraspis, and Cephalaspis. 



