226 TEN years' progress in vertebrate paleontology 



had been spent by each investigator in an effort to collect more and 

 better specimens, we might today have solved the puzzle of this form. 



Again: the problem of the Ostracophores stands scarcely less involved 

 today than a decade ago. It is true that the painstaking work of Traquair 

 on Ateleaspis indicates (Dollo) that this form is a connecting link be- 

 tween the groups of Thelodus and Cephalaspis; furthermore, the interest- 

 ing suggestion has been made (Dollo) that Drepanaspis was a blind 

 member of the Ostracophore class, having lived in soft ooze at shallow 

 depths. It was the knowledge of this form, by the way, which led to 

 the determination of the nature of Psammosteus (Woodward, Rohon). 

 On the other hand, the puzzle of Thelodus, Lasanius, and their kindred is 

 still unsolved. It is a plausible hypothesis that they stand not far from 

 the stock from which sharks descended, and that the plated forms came 

 into being in highly specialized lines. The work of Patten on Tre- 

 mataspis and on BotJirioIepis in this connection provides us with very 

 important documents for the understanding of, or, more fitly perhaps, 

 a confession of ignorance as to these early forms. Certain it is that we 

 can not place them today in closer relationships with true fishes. On the 

 other hand, we are convinced that the new facts do not demonstrate kin- 

 ship of these forms to Arachnids. Their peculiarities are better inter- 

 preted as tokens of high specialization in a line of forerunning chordates 

 in which, as yet, such structures as gillarches (and consequently true 

 jaws) had not been evolved. 



The studies of the past decade on Placoderms have been no more satis- 

 factory^ in solving the puzzle of the relationships of this group. The 

 structure of head and jaws has again been studied (Hussakof), and again 

 the anomalous characters of the "dentition" emphasized. In this regard 

 they certainly stood wide from the true fishes. In a general way we think 

 it safe to say that the phyletic gap which has separated Placoderms and 

 Pteryichthids tends to become smaller. But here again great divergence 

 exists in the opinions of the most eminent specialists. Eastman, studying 

 the dentition of Mylostoma, repeats his belief in the connection of these 

 forms with hmgfishes. Eegan. on the other hand, is convinced that 

 they are Teleostomes. Jaekel adheres to the view that they are masquer- 

 ading Chimaeroids. The writer has had the good fortune to see the 

 new materials from Wildungen, through Professor JaekeFs kindness, but 

 he is unable to see in them any evidence of this kinship ; these "fishes" 

 appear to represent a highly specialized line or series of lines of Arthro- 

 diran evolution, neither more nor less. In this connection a minor 

 puzzle has, we believe, been solved. Dollo shows, what we have already 

 suspected, that Ptyctodus is an Arthrodire, and Rhynckodus, which is 



