780 REPORT— 1900, 



and the base of the anal fin is considerably elongated. Platysomus has a still more 

 elongated anal fin, and the body is rhombic ; while in Chcirodus the body is still 

 deeper in contour, with peculiar dorsal and ventral peaks, long fringing dorsal and 

 anal fins, while the ventrals seem to have disappeared altogether. Here also, as in 

 the allied genus Cheirodopsis, the separate cyliudro-conical teeth characteristic of 

 the family are, on the palatal and spleuial bones, replaced by dental plates, remind- 

 ing us of those of the Dipnoi. Certainly the Platysomidse seem to me to form a 

 morphological series telling as strongly in favour of Descent as any other in the 

 domain of palaeontology. ^ 



If we njw return to the Palaeoniscida3 we find that they dwindled away in 

 numbers in the Jurassic rocks, and finally became extinct at the close of that 

 epoch. But already in the Lias (leaving the Triassic Catopteridfe out of considera- 

 tion for the present) we find that they have sent off another ofi'shoot sufficiently 

 distinct to be reckoned as a new and separate family, namely, the Chondrosteidte, 

 in which the path of degeneration, in all but the matter of size, seems to have been 

 entered on. 



In the genus C/iondrosteu/i, though the palajoniscid type is clearly traceable 

 in the cranial structure, there is marked degeneration as regards the amount of 

 ossification, and though the suspensorium is still obliquely directed backward the 

 toothless jaws are comparatively short, and the mouth seems now to have become 

 tucked in under the snout as in the recent sturgeon. Then the scales have entirely 

 disappeared from the skin except on the upper lobe of the heterocercal caudal fin, 

 where they are still found arranged exactly as in the Palseoniscida". 



Chondrosteus in fact conducts us to the recent Acipenseroids — the Poly- 

 odontidas (Paddle-fishes) and Acipenseridse (Sturgeons). 



The first of these resembles Cho7jdrosteus in the nakedness of the skin, except 

 on the upper lobe of the caudal fin,'^ tbe more palteoniscid aspect of the external 

 cranial plates, such of them as remain, for they are now still further reduced. But 

 in front of the mouth and eyes there is an addition in the form of an enormous 

 vertically flattened paddle-shaped snout covered above and below with a large 

 number of small ossifications. 



The sturgeons have, however, nearly altogether lost the palseoniscid arrange- 

 ment of the cranial roof-bones, which, strange to say, now exhibit an arrangement 

 reminding us of that in Dipferus, and the external facial plates are still more 

 reduced than even in Poh/odon ; but we may note a very strong resemblance to 

 Chondrosteus in the position of the mouth, the edentulous jaws, and the jugal 

 bone, indeed also in the palatal apparatus. 



So the sturgeons and paddle-fishes of the present day would seem to be the 

 degenerate, though bulky, descendants of the once extensively developed group of 

 PalaeoniscidDC, even as the modern Dipnoi are degenerated from those of Palaeozoic 

 times. 



We now notice another apparent offshoot of the Palreoniscidas, namely, the 

 family of CatopteridEe {Catopterus and Dictyopyge), which is limited to rocks of 

 Triassic age. Unfortunately the osteology of the head is not well known, but Dr. 

 Smith Woodward's observations are to the effect that both the head and shoulder- 

 girdle are of palreoniscoid type. The relationship of these small fishes to the 

 Palseoniscidfe is shown by the general shape, the number and position of the fins, 

 the rhombic ganoid scales, and the close arrangement of tbe rays of the median 

 fins. But the rays of the dorsal and anal fins are now almost equal in number to 

 their supporting ossicles, and the tail has become only abbreviate heterocercal. 

 That is to say, the caudal body prolongation no longer proceeds to the termination 

 of the upper lobe, which is reduced in size and in the number of its rays. The 



' R. H. Traquair, ' Structure and Affinities of the Platysomidfc,' Trans. Eoy. Soc. 

 Edin. ssix. 1879, pp. 343-301. 



- Collinge has, however, found rudimentary scales in the skin of the recent 

 Polydon folium (Journ. A^iat. and PTiys. ix. pp. 485-4S7), and Cope has described an 

 allied Eocene genus, Crossopholis, in which minute scales are seen {Mem. JS'^at. Acad, 

 Sciences, n\. 18S6, pp. lGl-iq3). 



