xxxi 



tinned to grow together, ejecting more and more the mesopterygium, which 

 would become, pari passu, correspondingly elongated and extended back- 

 wards ; finally, it would become segmented, and the actinosts and rays 

 having become lateral instead of terminal, the limb of Ceratodus would be 

 developed. 



(3) And finally, should the lateral elements and rays of the pectoral fin 

 of Ceratodus become (1) successively aborted, and finally (2) entirely atro- 

 phied, the limbs of (1) Lepidosiren and (2) Protopterus would be repro- 

 duced. 



In view of the varying combinations of the basal elements of the limbs 

 in the Elasmobranchiates (e. g., Scymnidse, in which all are consolidated), 

 the suppositions thus hazarded do not appear to be unreasonable or op- 

 posed by histological or developmental principles or facts. 



The question, how the limbs of the quadruped Batrachians have become 

 specialized from such members, is foreign to the present inquiry. 



HYOGANOIDS. 



The question now recurs, what are the relations and nearest of kin of 

 the Hyoganoids ? 



A more significant hint appears to be furnished by the structure and form 

 of the scales of some of the representatives of the group, than by any other 

 part of the structure. 



The similarity, in form as well as in intimate structure, of the scales of 

 t~e Lepidosteids to those of the Polypterids is so close, and the peculiari- 

 ties and specialized characters of those scales are so many, that the fishes 

 distinguished by such common characters must have inherited them from a 

 common progenitor. Any other supposition would be in opposition to 

 the strongest probabilities. For it must be remembered that the commu- 

 nity of character is not a general one like that between ordinary cycloid 

 or ctenoid scales, but a close one of a very specialized and proportionately 

 suggestive nature. This similarity is also coincident with a corresponding 

 though not so great similarity of the skull, especially the suspensorial 

 apparatus, etc. 



But while the Polypterids have deviated widely in some respects and, 

 among others, in their limbs and the connections of the air-bladder and in- 

 testinal canal from the primitive stock, the Lepidosteids, deviating equally 

 in other respects, have done so less in respect to their limbs. 



In the Lepidosteids and the Amiids are found the nearest representatives, 

 among the Ganoids, of the line of descent in the direction of the typical 

 fishes, as in the Crossopterygians and Dipnoans are found the nearest 

 living forms in the line leading towards the Batrachians and higher Verte- 

 brates. 



