September 20, 1900] 



NATURE 



507 



to M. DoUo for his brilliant essay, and though we must agree 

 with him in many things, such as that the Crossopterygii were 

 not derived /r^w the Dipnoi, and that the modern representatives 

 of the latter group are degenerate forms, yet as to the immediate 

 ancestry of the Dipnoi themselves, and the diphyletic origin of 

 the so-called archipter}'gium, we had best for the present keep 

 an open mind. 



In his "Catalogue of the Fossil Fishes " in the British Museum 

 (vol. ii. 1891), Dr. Smith Woodward, following the suggestion of 

 Newberry in 1875, classified the Coccosteans or "Arthrodira" 

 as an extremely specialised group of Dipnoi. At first I was 

 much taken with that idea, but after looking more closely into 

 the subject I began to doubt it extremely. My own opinion at 

 present is that the Coccosteans are Teleostomi belonging to the 

 next order, Actinopterygii ; but Prof. Bashford Dean, of New 

 York, will not have them to be even " fishes," but places them 

 in a distinct class of "Arthrognatha," which he places next to 

 the Ostracophori ( = Ostracodermi), even hinting at a possible 

 union with them, whereby the old " Placodermata " of McCoy 

 would be restored. It will, therefore, be better to leave them 

 out of consideration for the present, pending a thorough re- 

 €xamination of their structure and affinities. 



We come then to the great order of Actinopterygii, to which 

 a large number of the fishes of later Palreozoic age belong, as 

 well as the great mass of those of Mesozoic, Tertiary and 

 Modern times. Of these we first take into consideration the 

 oldest sub-order, namely, the Acipenseroidei or Sturgeon tribe, 

 in which the dermal rays of the median fins are more numerous 

 than their supporting ossicles, while the tail is, in most, com- 

 pletely heterocercal. And the oldest family of Acipenseroids 

 with which we are acquainted is that of the PalceoniscidM, 

 which, in addition to well-developed cranial and facial bones, 

 has the body normally covered with rhombic ganoid scales 

 furnished with peg-and-socket articulations. Of this family one 

 genus, Cheirolipis, appears in the same Devonian strata 

 (Orcadian series) with the earliest known Crossopeterygii, and 

 of its immediate ancestry we know no more than we do of theirs. 

 Cheirokpis is a fully evolved palteoniscid, as shown by its 

 oblique suspensorium, wide gape, and other points of its structure. 

 In the Lower Carboniferous rocks of Scotland, where the family 

 attains an enormous development, we find one or two genera, e.g. 

 Canohius, which appear less specialised, as the suspensorium is 

 nearly vertical, and the mouth consequently smaller. 



This family endures up to the Purbeck division of the Jurassic 

 formation, and in the Carboniferous Cryphiolepis, the Lower 

 Permian Trissolepis and the Jurassic Coccolepis we find the 

 same degeneration of the rhombic scales into those of a circular 

 form and imbricating arrangement, which we find repeated in other 

 groups of "Ganoids." In fact, in one Carboniferous genus, 

 Phanerosteon, the scales disappear altogether with the exception 

 of those on the body prolongation in the upper lobe of the caudal 

 fin, and a few just behind the shoulder-girdle. 



And in these Palaeozoic times we notice also a .side branch of 

 the PalDeoniscidae, constituting the family Platysomid?e, in which, 

 while the median fins acquire elongated bases, the body becomes 

 shortened up and deep in contour. The scales become high and 

 narrow, their internal rib and articular spine coincident with the 

 anterior margin ; the suspensorium, too, instead of swinging 

 back as in the typical Palseoniscidae, tends to be directed 

 obliquely forward, while the snout becomes simultaneously 

 elongated in front of the nares. 



A most interesting series of forms can be set up, beginning 

 with Eurynotus, which, though it has the platysomid head 

 contour and a long-based dorsal, has only a slight deepening oi 

 the body, and still retains the palaioniscid squamation and a 

 short-based anal fin. In Mesolepis, which resembles Eurynotus 

 in shape, being only slightly deeper, we have now the charac- 

 teristic platysomid squamation, 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 Cheirodits 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. Heie also, as in the allied genus 

 Cheirodopsis, the separate cylindro-conical teeth characteristic of 

 the family are, on the palatal and splenial bones, replaced by 

 dental plates, reminding us of those of the Dipnoi. Certainly 

 the Platysomidre seem to me to form a morphological series 

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

 domain of palaeontology.^ 



1 R. H. Traquair, " Structure and Affinities of the Platysomidae," Trans. 

 Roy. Soc. Eitin., xxix. 1879, pp. 343-391. 



NO. 1612, VOL. 62] 



If we now return to the Palaeoniscidse 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 Catopteridae out of consideration for 

 the present) we find that they have sent off another offshoot 

 sufficiently distinct to be reckoned as a new and separate family, 

 namely, the Chondrosteid?e, in which the path of degeneration, 

 in all but the matter of size, seems to have been entered on. 



In the genus Chotidrosteus, though the palaeoniscid 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 tooth- 

 less 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 

 Palneoniscidas. 



Chondrosteus in fact conducts us to the recent Acipen- 

 seroids — the Polyodontidae (Paddle- fishes) and Acipenseridse 

 (Sturgeons). 



The first of these resembles Chondrosteus in the nakedness of 

 the skin, except on the upper lobe of the caudal fin,i the more 

 palceoniscid 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 

 palteoniscid arrangement of the cranial roof-bones, which, 

 strange to say, now exhibit an arrangement reminding us of 

 that in Dipterus, and the external facial plates are still more 

 reduced than even in Polyodon ; 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 Palajoniscidse, even as the 

 modern Dipnoi are degenerated from those of Palaeozoic 

 times. 



We now notice another apparent offshoot of the Paloeoniscidse, 

 namely, the family of Catopteridae [Catoptertis and Dictyopyge)^ 

 which is limited to rocks of Triassic age. Unfortunately the 

 osteology of the head is not well known, but Dr. Smith Wood- 

 ward's observations are to the effect that both the head and 

 shoulder-girdle are of palaeoniscoid type. The relationship of 

 these small fishes to the Palaeoniscidae is shown by the general 

 shape, the number and position of the fins, the rhombic ganoid 

 scales, and the close arrangement of the 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 

 Catopteridae are obviously an annectent group, as although from 

 their abbreviate heterocercal tail they have usually been placed 

 in the next sub-order. Dr. Smith Woodward prefers to look 

 upon them as Chrondrostei {i.e. Acipenseroidei).^ Wherever we 

 place them they express the beginning of a set of changes 

 towards a more modern type of fish, which are emphasised in 

 the great series of Lepidosteoid fishes (Protospondyli -f ^Etheo- 

 spondyli of Smith Woodward), being the fishes more or less 

 allied to the recent Bony Pike of North America. 



But these changes must have been well advanced before the 

 Triassic era, for already in the Upper Permian occurs the genus 

 Acentrophorus, whose fellowship with Semionotus, LepidotuSy 

 and all the rest of the series of Mesozoic semi-heterocercal 

 " Ganoids " is at once obvious. 



If we look at the configuration of a typical Jurassic member 

 of this series, such as Lepidotus or Etignathus, we shall at once 

 see that we are a stage nearer the modern osseous fish. Though 

 the scales are bony, rhombic, and ganoid we are struck by the 

 " Teleostean "-like aspect of the external bones and plates of 



1 CoUinge has, however, found rudimentary scales in the skin of the recent 

 Polytton folium {/onrn. Anat. and Phys., ix. pp. 458-487), and Cope has 

 described an allied Eocene genus, Crossopholis, in which minute scales are 

 seen (.l/c/«. Nat. Acad. Sciences, iii. 1886, pp. 161-163). 



^ Dr. Smith Woodward also refers the singular Belonorhynchidse of the; 

 Trias to the same sub-order on account of the excess of the number i _f the 

 dermal rays of the dorsal and anal over that of their supporting ossicles, 

 even although the tail is here abbreviate diphycerc.al. 



