S 4 SCIENCE PROGRESS. 



lung-sac of the Dipnoi is much more lung-like and their 

 atrial septum is more complete than that of some Urodeles, 

 but the probability that this simplification may be the 

 outcome of a substitution of organs, with accompanying 

 structural degeneration, must not be overlooked. What- 

 ever the truth, the Dipnoi stand alone among fishes in 

 the possession of the septum auricularum and a pulmonary 

 vein and posterior nostril ; and not the least important 

 desideratum concerning them, is the settlement of the 

 question whether the latter is or is not the homologue 

 of that of the terrestrial vertebrata. The belief in the 

 so-called archiptcrygial characters of the paired fins of 

 Ceratodus has not met with general acceptation ; and 

 although the " archiptcrygium " theory is still upheld by 

 palaeontologists, the detailed characters of the paired 

 fins of the Palaeozoic Selachians Cladodus and the Xena- 

 canthidce are by no means readily reconcilable with those 

 of the Ceratodtis type. 70 And not the least formidable 

 difficulty which besets their argument is the fact that the 

 biserial condition is not forthcoming in the pelvic member 

 of these old elasmobranchs. There are not wanting those 

 who, with Haswell, 71 believe that "the limb of Ceratodus, 

 so far from representing a primitive and generalised type, is, 

 as indeed we should expect from various other points in the 

 organisation of the animal, in reality highly specialised". 



Professor Semon's previous work ranks high among 

 contemporary scientific investigation, and, if only in con- 

 sideration of the above-cited difference of opinion on a 

 subject so important to the vertebrate morphologist as 

 that which he now has in hand, his work, striking- as it 

 does at the very foundations of one of the most central 

 groups in the vertebrate sub-kingdom, cannot fail to reveal 

 facts of a revolutionary order. The two short chapters 

 of his treatise thus far published are largely prefatory 

 in nature. They open with a history of the genus, and 

 an account of its distribution and habits. The author 

 controverts the assumption of the animal's alleged migra- 

 tion on land, and confesses himself, like Spencer, 72 unable 

 to find evidence of the imagined summer sleep, presup- 



