MOEPHOLOGICAL METHOD AND RECENT PROGRESS IN ZOOLOGY. 593 



and Pi'otoptcrii^, with its 30 to 85, us varieties of a specie's, it is the 

 more interesting- to tindthat the Congo has hitely 3'ielded a Pi'otoptrinm 

 {P. Dollof) with the k^pidosiren rib fonnida, viz, 54: pairs. 



As a foremost result of American paleontok)oical research we have 

 to record the occurrence, in the Devonian of Ohio, of a series of 

 colossal fishes known as the Arthrodira, the supposed dipnoan affinities 

 of which are still a matter of douljt. 



A\^e have evidence that the osseous skeleton in a plate-like form first 

 appeared as a protection for the eye of a primitive shark, and coming- 

 to recent forms having special ])earings on the teachings of the rocks, 

 we have to acknowledge the capture in the Japanese seas of a couple 

 of ancient sharks, of which one {CladoNelachus), since ol)served to 

 have a distribution extending to the far north, is a survivor from 

 Devonian times; the other {Mlt,suhur/na)^ a genus whose grotesque- 

 ness leaves no dou])t of its identity with the Cretaceous lamnoid 

 ^Sca pernor hyncJiits. In the elucidation of the Sturiones, and the deter- 

 mination of their affinities with the ancient Paheoniscida^ a master 

 stroke has been achieved. In the Old Red genus l^ahv.osipomlylKS we 

 have become familiar with an unmistaka])le marsipobranch, possessing, 

 as do certain living fishes, a notochord, anmdated, but not vertebrated 

 in the strict sense of the term. The climax in Ichthyopaheontology, 

 however, has been reached in the discovery of Silurian foi'ms, which 

 there is every reason to believe explain, in an unexpected wa}^ the 

 hitherto anomalous Pteras- and Cephalaspidians, by involving them 

 in a conununity of ancestry with the primitive Elasmobranchs. The 

 genera Thehxlus^ Dn-panaspis, Afeleaspls, and Zanarkh, chief among 

 these annectant and ancestral forms, are among the most remarkable 

 vetc})rate fossils known. 



Passing to the Recent fishes alone, the discovery which must take 

 precedence is that of the mode of origin of the skeletogenous tissue of 

 their vertebral column. The fishes, unlike all the higher Vertebrata, 

 have, when young, a notochord invested in a dou])le sheath, there 

 being an inner chordal sheath, an outer cuticular, which latter is alone 

 present in all the higher groups. The skeletogenous cells, by whose 

 activit}' the cartilaginous vertebral skeleton is formed, arise outside 

 these sheaths; but whereas when proliferating they in one series 

 rcmtiin outside, they in the other, ])y the rupture of the cuticular 

 sheath, invade the chordal. This distinction enal>les us to discrimi- 

 nate l)etwcen a Ch/mlal series, which embraces the Cliinueroids, Elas- 

 mobranchs, and Dipnoi, and a Perickordal, consisting of the Teleosts, 

 Ganoids, and Cyclostomes. 



In consideration of the enormity of the structural gap between tlio 

 Cyclostomes and the higher Vertebrata this is an extraordinar3M'esult. 

 For be it remembered that, in addition to their well-known characters. 



SM 1902-^ 38 



