128 — RELATIONSHIPS OF LUNG-FISHES 
have now been closened by the proof that their paddle- 
shaped fins may be directly deduced from a “ monoserial 
archipterygium,” and that their diphycercal caudal, formerly 
regarded as most primitive in plan, may have been acquired 
secondarily after a condition of heterocercy (W. N. Parker, 
Traquair, Dean). 
The resemblances of Dipnoans to Elasmobranchs might © 
be summarized in the following structures: — , 
I. VERTEBRAL Axis. Its notochordal condition and 
simple metameral, neural, and hemal elements suggest the 
conditions of Cladoselache (p. 80); in that ancient form, 
however, the vertebral processes had not come into rela- 
tion with the unpaired fins. 
II. Sxutzt. The chondrocranium is as yet largely re- 
tained; as yet no dentigerous membrane bones of the 
mouth rim (maxillary and premaxillary) have appeared. 
III. Trern. These are clearly of an elasmobranchian 
order; the tubercles of the dental plates (Fig. 125) suggest 
closely a shagreen pattern; in Phaneropleuron, marginal 
cusps have even been retained. The palatine and splenial 
plates parallel strikingly some of the forms of Cestraciont 
dentition. 
IV. Brain. Its structures are of an advancing elasmo- 
branchian order, annectent with reptilian (Ceratodus) and 
amphibian types (Protopterus).* 
V. VISCERAL CHARACTERS. Heart, gills, digestive tract, 
vessels, mesenteries. . - 
The closely corresponding characters of Phaneropleuron 
and Pleuracanthus might be looked upon as independently 
acquired; but in view of the many nearnesses of their 
phyla, these characters may reasonably be regarded as 
proof of genetic kinship. 
* Burckhardt, 
