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TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



exception to the law of the irreversibility of evolution. It is rather a special application of 

 that law, which, as originally enunciated by Dollo, recognized clear evidence for changes 

 in the direction of evolution. 



In the sagittal section of the skull of the puffer Lagocephalus (Fig. 168) we notice that 

 the tip of the upper beak is almost in horizontal line with the basis cranil and the para- 

 sphenoid is horizontal or even tilts slightly upward, whereas in the small-mouthed nibbling 

 fishes such as the teuthids and balistids, which I am supposing to represent the remote 

 ancestors of the puffers, the upper jaw is far beneath the level of the basis cranii and the 



Lagocephalus laevigatas 



Fig. 168. Lagocephalus lizvigatus. Right half of skull viewed from within. 



parasphenoid is directed obliquely downward. Four factors have probably taken a leading 

 part in producing this result. In the first place, as I have suggested above, there has been 

 a marked shortening of the preorbital face length, this in itself bringing the dorsal border 

 of the mouth up to a higher level. Secondly, there has probably been a marked decrease 

 in the total height from the summit of the back to the lower border of the throat when a 

 primitive high-bodied fish was changed into a shallow-bodied and secondarily elongate 

 type. Thirdly and almost indissolubly connected with the last factor, there has been a 

 great secondary increase in transverse diameters, finally producing the sub-spherical con- 

 tours of the porcupine fishes. This brings into view the fourth and perhaps leading factor 

 of all, the modifying influence of the dominant "puffing" apparatus in both the puffers 

 and the porcupines. 



In the mechanism of inflation, as so clearly exposed by Parr (1927c); the broad 

 obliquely-placed muscles of the body-wall force water or air into the oesophageal diver- 



