202 



SCIENCE 



[N. S. Vol. XXVII. No. 684 



past year has unravelled the problem of 

 the origin of the vertebrata. And I fear 

 that we can, as usual, report nothing better 

 than progress. To our infinite regi'et there 

 have been forthcoming no new results on 

 the Silurian fishes of Scotland, the clan of 

 Thelodus, Lanarkia, Lanasius. We recall, 

 however. Dr. Patten's splendid work in 

 unearthing fins, tail and other structures 

 of the lowly Boihriolepis, and the beau- 

 tiful models which he demonstrated before 

 the Zoological Congress in Boston. Never- 

 theless, he has not established, it seems to 

 me, his thesis as to an arthropodial 

 chordate. But he has given us valuable 

 perspective as to the position of this very 

 early "fish": for his studies show clearly 

 how remote a cousin was this form to any 

 type of -our kno-wn chordates. On evidence 

 which he adduces, such as the position of 

 the anus between the horizontal postero- 

 ventrolateral plates, curious little sepa- 

 rately set jaws, which may have op- 

 erated laterally instead of vertically, and 

 in other regards, we can only conclude that 

 Boihriolepis belonged to a terminal group, 

 as far, at least, as our living fishes are con- 

 cerned. By no stretch of my morpholog- 

 ical imagination can I see how this cara- 

 paced, hinge-spined, and thread-tailed 

 anomaly could have given birth to a line of 

 our real backboned animals. 



Important in this connection is a sum- 

 mary of Hussakof which shows in how 

 many regards the group to which Boihrio- 

 lepis belongs corresponds, as earlier writers 

 believed and as later writers denied, to the 

 group of Arthrodira containing such forms 

 as Coccosteus, Dinichthys and Titanich- 

 thys. 



In short, we may be, dealing in these 

 groups collectively, which are popularly 

 known as ' ' placoderms, ' ' with a great line, 

 a phylum or subphylum, of chordate crea- 

 tures which preceded the types of modern 

 chordates and which in spite of lines of 



heroically developed tribes, families, genera 

 and species, died out as the modern chor- 

 dates came into competitive being. They 

 were, I suggest, chordates in which there 

 were misdirected, or more accurately, un- 

 fortunate, evolutional tendencies, affecting 

 structures or correlated combinations of 

 structures. These may well have carried 

 the placoderms along successfully to a cer- 

 tain point, but beyond this basis their 

 morphological restrictions did not permit 

 them to go. Thus these creatures may 

 have been chordates which were defective 

 in the substratum of a gill arch type 

 of mouth and they had not, therefore, 

 laid the necessary foundation for the endo- 

 facial complex of the higher animals: 

 I mean that the mouth region of these an- 

 cient forms had not the capability of at- 

 taining the strengthening support of endo- 

 skeletal elements, the greater mobility 

 which gill muscles provided, the greater 

 vascular and nervous supply, the fuller 

 channel for sensory impressions : instead, 

 in those pioneer forms, the mouth appa- 

 ratus was fashioned on a simpler, more 

 independent and therefore shorter-lived 

 plan. Their jaws were strictly dermal ele- 

 ments, and operated by dermal muscles- 

 all in all a mouth mechanism non-homol- 

 ogous with that of the higher vertebrates. 



In the great group of the placoderms 

 known as Arthrodira, including such puz- 

 zles as Dinichihys, Biplognaihiis, Mylo- 

 sioma and Coccosteus these jaws seem to 

 have run a gamut of adaptive changes: 

 then followed a period of extermination, 

 for we know to-day of no placoderm — ceph- 

 alaspid, pterichthyid, coccosteid— which 

 passed an undisputed boundary line into 

 the Carboniferous. The ancient tree died 

 and its branches di'opped off. But be- 

 fore its extinction some of its members 

 developed curiously specialized forms and 

 structures, paralleling the characters of 

 modern fishes, as though in a final and 



