276 



HARD WICKE 'S S CIENCE- G OS SI P. 



lakes, is the sole member of the sub-order Ainiadu:. 

 The Lepidosteidis include the living bony pikes, 

 inhabitants of the rivers of the same continent, and 

 fossil forms in all the formations reaching back to the 

 Devonian. There remains for disscussion but the 

 sub-order Crossopterygidtr, that important group of 

 fringe-finned ganoids, through which Professor 

 Huxley considers the passage from the fishes to the 

 reptiles took place. All the families of this well- 

 defined sub-order are characterized by the possession 

 of two dorsal fins, and by lobate paired fins having a 

 central axis or stem covered with scales like the body 

 walls, and surrounded by a fringe of fin rays. Jugular 

 plates always replace the branchiostegal rays, and the 

 scales are either rhomboidal or cycloidal. The 

 families Saurodiptenni, Glyptodipterini, and PJianero- 

 pleurini are restricted to the Palaeozoic rocks. The 

 Calacaiithiiii range from the Carboniferous to the 

 Chalk, and the Polypterini, comprising only the living 

 Polyptents and Calamokhihys of Africa, alone repre- 

 sent this numerous race of fishes at the present day. 

 Tlie genus Polyptents is remarkable for the unique 

 arrangement of its subdivided dorsal fin, and by the 

 possession of a double cellular air-bladder, which 

 most nearly approximates to the true lungs of the 

 Dipnoi. It has least structural affinities with the 

 Cxlacaiiths, its nearest allies in time, and is most 

 closely zoologically related to the rhomboidal scaled 

 Saurodipterines of the Devonian, from which it is 

 separated by an enormous gulf of geological time, as 

 no intermediate links have been discovered. In the 

 notochordal Phaneroplciirini we find forms which 

 most closely resemble the acutely lobate-finned 

 Lepidosiren. The shape of the body, number, position, 

 and structure of the fins, and all the elements of the 

 internal skeleton, exactly foreshadow those of the mud 

 fishes. Like them Phaneropleuron was covered with 

 thin cycloidal scales, through which the long and well- 

 ossified ribs show so plainly in the fossil state as to 

 suggest the name of the genus. The dentition, how- 

 ever, differs from that of Ceratodus and Lepidosiren, 

 being composed of a row of short conical teeth in 

 each jaw, and in the absence of the grooved dental 

 plates so characteristic of the tnie Dipnoi, it is uncertain 

 whether this family can be associated with the other 

 members of that order. The chain of descent is 

 carried on by the Caacanth'ini, the only fringed-finned 

 ganoids occurring in the mesozoic rocks. They can 

 be traced up from Calacantkics, in the Carboniferous, 

 through Holophagiis in the Lias and Undina in the 

 Oolites, up to Macropoma in the Chalk. The family 

 is distinguished by cycloid scales, hollow fin-supports, 

 and a notochordal skeleton built on the same principle 

 as that of the mud fishes. In some genera the walls 

 of the air-bladder are ossified. This peculiarity, 

 which was first suspected by Mantell, is especially 

 remarkable in Undina and Macropot/ia. No fossil 

 Crossopterygids have been discovered in Tertiary 

 strata, but it is the opinion of Professor Huxley that, 



as the rhomboidal scaled Saurodipterines of the 

 Devonian rocks are now represented by the living 

 Polypterus, so the stiff"- walled lungs of the Lepidosiren 

 are the homologues of the ossified air-bladder of the 

 Ccelacanths ; and thus that genus carried up the 

 cycloidal branch of the Crossopterygids to the present 

 day. Such, in the abstract, is the life-history of fishes, 

 a class characterized, like other divisions of the animal 

 kingdom, by the extinction of some groups after a 

 brief existence, and by the persistent endurance of 

 others through untold ages. In the few genera of 

 living ganoids we have undoubtedly the surviving 

 descendants of a numerous and powerful race, which 

 prevailed in the Devonian epoch, and by the discovery 

 of fossil dipnoal forms, the progenitors of Ceratodus 

 and Lepidosiren, the Dipnoi are likewise proved to be 

 of ancient lineage. The greater part of the existing 

 piscine fauna, on the contrary, is shown to be of 

 comparatively modern date. Moreover, in considering 

 the fact that the early fishes are remarkable from a 

 combination of diverse characteristics which subse- 

 quently become the distinguishing peculiarities of 

 distinct families, and of a higher order, we find 

 further evidence that the ancient ganoids formed the 

 parent stocks from which the succeeding fishes, 

 amphibians, and reptiles have diverged. In some 

 sauroid Devonian fishes the position and structure of 

 the teeth foreshadow those of the Labrinthodont 

 reptiles ; in others the throat is protected by gular 

 plates, 'a fashion retained in the Carboniferous 

 amphibia. Again, in some species the scales are 

 surface-pitted, like the scutes of crocodiles. While, 

 in the notochordal weak-limbed amphibians of the 

 coal-measures, with minute body-scales, and partly 

 osseous skulls, we cannot fail to recognize structural 

 peculiarities now found in the swamp-dwelling mud 

 fishes. Thus in the anomalous "scaled sirens" we 

 have the "persistent type" of an ancient group of 

 fishes, in which now, as in the old time, the piscine 

 and amphibian characters are so united as to com- 

 pletely efface the line of demarcation between the 

 orders, and effectually link the fishes to the reptiles. 



MICROSCOPY. 



Cleaning Glass Slides. — I am not sure whether 

 the following method of cleaning used glass slides 

 and covering glasses has been mentioned before in 

 your columns ; if it has I can bear testimony to its 

 utility. I had tried previously to remove the har- 

 dened balsam in many ways, and had succeeded fairly 

 with a mixture of prepared chalk, methylated spirit, 

 and liquid ammonia, but found this objectionable be- 

 cause it was such a dirty job. I now simply warm 

 the slides over a flame, and push ofT the covers into 

 strong sulphuric acid (oil of vitriol), and leave them 

 therein for a short time ; when clean, I drain off, and 

 rinse with a little fresh acid, and finish off by washing 



