EASTMAN : STRUCTUKE AND EELATIONS OF MYLOSTOMA. 19 



terial whether Arthrodires were autostylic or hyostylic; tlie ultimate 

 proof of their Dipuoan relationships must rest on other characters than 

 these. Fiirbringer's commentary ou the views just stated will be 

 found on page 501 of his monograph on the skeleton of Dipnoans to 

 which we have several times referred. 



Body characters indicating Dipnoan affinities of Arthrodires. — It 

 is safe to affirm that the main outlines of the skeletal structure of 

 Arthrodires, except only for the development of dermal armor, agrees 

 intimately with those of recent Dipnoans. A certain amount of resem- 

 blance might be explained as due to pai'allelism ; but no such theory 

 can account for the striking coincidence in structural plan to be observed 

 throughout all pai'ts of the body. It would be absurd to suppose that 

 one group of organisms, such as Arthrodires, coincides fortuitously with 

 the principal features of another group, — as, for example, Ceratodonts 

 — without the two being nearly related. When it is realized that 

 essential unity of structural type pervades not only the cranium but 

 the entire skeleton of the groups mentioned, the obvious inference to be 

 drawn is that they shai'e a common origin. This is a legitimate, and, 

 indeed, unavoidable deduction. We might even proceed further, and, 

 if one cared to speculate as to the ultimate origin of Dipnoans, a num- 

 ber of characters would be found suggesting descent from Pleuracanthus- 

 like sharks.^ 



The body characters in which Arthrodires may be claimed chiefly to 

 resemble modern Dipnoans are as follows : (1) a persistent notochord ; 

 (2) dipliycercal tail ; (3) segmental correspondence between tlie skele- 

 tal supports of the soft dorsal fin and the vertebi'al axis, to the extent 

 that the two sets of interspiuous bones are articulated with each other 

 and also with the neurapophyses by expanded exti'emities, there being 

 an equal number of interneurals and neural spines ; (4) punctate dermal 

 plates ; and (5) — although this last point requires further confirma- 

 tion — an apparently similar conformation of the pelvic arch. 



On the other hand, the following points of difi'erence are to be noted 

 between Arthrodires and modern lung-fishes: (1) encasement of the 

 anterior portion of the trunk in dermal armor ; (2) apparent atrophy of 

 the anal and pectoral fins; and (3) shortening of the dorsal into a 

 single, abbreviate, membranous fin situated in the middle of the back. 

 None of these characters, however, are of fundamental importance, 



1 Inter alia, the notochordal axis, diphycercal tail, biserial pectoral fins, basi- 

 pterygial pelvic girdle, and, most singular of all, the Dipnoan-like arrangement of 

 dermal bones roofing the head. 



