Subclass Dipneusti, or Lung-fishes 619 



are shown ; in the brain region the medullary folds are still 

 slightly separated. 



In an older embryo the fish-like form may be recognized. 

 The medullary folds have completely fused in the median line, 

 and the embryo is coming to acquire a ridge-like prominence; 

 optic vesicles and primitive segments are apparent, and the 

 blastopore appears to persist as the anus. The continued 

 growth of the embryo above the yolk mass is apparent ; the head 

 end has, however, grown the more rapidly, showing gill-slits, 

 auditory, optic, and nasal vesicles, at a time when the tail mass 

 has hardly emerged from the surface. Pronephros has here 

 appeared. It is not until the stage of the late embryo that the 

 hinder trunk region and tail come to be prominent. The em- 

 bryo's axis elongates and becomes straighter; the yolk mass 

 is now much reduced, acquiring a more and more oblong form, 

 lying in front of the tail in the region of the posterior gut. The 

 head and even the region of the pronephros are clearly separate 

 from the yolk-sac ; the mouth is coming to be formed. 



According to Eastman (Ed. Zittel), the skeleton of Xco- 

 ceratodus is less developed and less ossified than that of its 

 supposed Triassic ancestors. A similar rule holds with regard 

 to the sturgeons and some Amphibians. 



Lepidosirenidae. The family Lepidosirenida;, representing the 

 suborder Diploneumona, is represented by two genera of mud- 

 fishes found in streams of Africa and South America. 

 Lepidosiren paradoxa was discovered by Natterer in 1837 in 

 tributaries of the Amazon. It was long of great rarity in 



FIG- 388. Adult male of Lepidosiren paradoxa Fitzinger. (After Kerr.) 



collections, but quite recently large numbers have been ob- 

 tained, and Dr. J. Graham Kerr of the University of Cambridge 

 has given a very useful account of its structure and develop- 

 ment. From his memoir we condense the following record 

 of its habits as seen in the swamps in a region known as Gran 

 Chaco, which lies under the Tropic of Capricorn. These 'swamps 



