September 20, 1900] 



NA TURE 



503 



You are all already acquainted with the telling evidence in 

 favour of Evolution furnished by the well-known Mammalian 

 limbs, as well as of teeth, in which the progress, in the course of 

 time, from the more general to the more special is so obvious that 

 I cannot conceive of any unprejudiced person shutting his eyes to 

 the inference that Descent with modification is the reason of these 

 things being so. Suppose, then, that on this occasion we take 

 up the palaeontological evidence of Descent in the case of fishes. 

 This I do the more readily because what original work I have 

 been able to do has lain principally in the direction of fossil 

 ichthyology ; and again, because it does seem to me that it is 

 in this department that one has most reason to complain of 

 want of interest on the part of recent biologists, even, I may 

 say, of some professed paleontologists themselves. 



But the subject is really of so great an extent that to exhaust 

 it in the course of an address like the present would be simply 

 impossible, so I shall in the main limit myself to the considera- 

 tion of Palreozoic forms, and this more especially seeing that we 

 may hope for a large addition to our light on the fishes of the 

 more recent geological formations from the fourth volume of the 

 "Catalogue of Fossil Fishes" in the Briti-h Museum, which 

 will soon appear from the pen of my friend. Dr. A. Smith Wood- 

 ward. I need scarcely say how much his previous volume has 

 conduced to a better knowledge of the Mesozoic forms. 



Here I may begin by boldly affirming that I include the 

 Marsipobranchii as fishes, in spite of the dictum of Cope that no 

 animal can be a fish which does not possess a lower jaw and a 

 shoulder-girdle. Why not ? The position seems to me to be a 

 merely arbitrary one ; and it is, to say the least, not impossible 

 that the modern Lampreys and Haggsmay be, as many believe, 

 the degenerate descendants of originally gnathostomatous forms. 



To the origin of the Vertebrata, Palceontology gives us no clue, 

 as the forerunners of the fishes must have been creatures which, 

 like the lowest Chordata of the present day (Urochorda, Hemi- 

 chorda, Cephalochorda), had no hard parts capable of preserva- 

 tion. And though I shall presently refer again to the subject, I 

 may here affirm that, so far as I can read the record at least, it 

 is impossible to derive from Palteontology any support to the 

 view, recently revived, that the ancient fishes are in any way 

 related to Crustacean or merostomatous ancestors. 



What have we then to say concerning the most ancient fishes 

 with which we are acquainted ? 



The idea that the minute bodies, known as Conodonts, which 

 occur from the Cambrian to the Carboniferous, are the teeth of 

 fishes and possibly even of ancient Marsipobranchs may now be 

 said to be given up. They are now accepted by the most trust- 

 worthy authorities as appertaining to Invertebrata such as 

 Annelidcs and Gephyrea. 



More recently, however, Rohon^ has described from the Lower 

 Silurian of the neighbourhood of St. Petersburg small teeth 

 {Palaeodus and Archodns) associated with Conodonts, and which 

 seem to be real fish teeth, but not of Selachians, as is shown by 

 the presence of a pulp cavity surrounded by non-vascular 

 dentine. It is impossible to say anything more of their 

 affinities. 



Obscure and fragmentary fish remains have been obtained by 

 Walcot, and described by Jaekel, from rocks in Colorado sup- 

 posed to be of Lower Silurian or Ordovician age.^ But doubts 

 have been thrown on their age, and the fossils themselves, 

 which have, it must be owned, a very Devonian look about 

 them, are so extremely fragmentary that they do not help us 

 much in our present purpose. 



It is not till we come to the Upper Silurian rocks that we 

 begin to feel the ground securely under our feet, though we may 

 be certain, from the degree of specialisation of the forms which 

 we there find, that fishes lived in the waters of the globe for 

 long ages previously. 



Characteristic of the " Ludlow bone-bed " are certain minute 

 scales on which Pander founded the family Ccelolepidte, having 

 a flat or sculptured crown, below which is a constricted "neck," 

 and then a base u'.ually perforated by an aperture leading into a 

 central pulp cavity. As these little bodies, looked upon by 

 Agassiz as teeth, were shown by McCoy to be scales, and as 

 they occurred at Ludlow in England and Oesel in Russia along 

 with small Selachian spines {Onc/ins), they were usually con- 

 sidered as appertaining, with the latter, to small Ceslraciont 

 sharks. The genera Thelodus, Coelokpis and others were 



1 " Ueber untersilurische Fische," MHanges CM. et Paliont. vol. i. (St. 

 Petersburg, 1899), pp. 9-14. 

 - Bulletin Geol. Soc. America, vol. iii. 1892, pp. 153-171. 



NO. 1612. VOL. 62] 



founded on these dermal bodies, but it is doubtful if any but the 

 first of these names will stand. 



But the aspect of affairs was altogether changed by the dis- 

 covery three years ago, by the officers of the Geological Survey, 

 of entire specimens of Thelodus in the Upper Silurian rocks of 

 the South of Scotland, from which it was evident that the fish,, 

 though somewhat shark-like, could hardly be reckoned as a true 

 Selachian.^ 1 helodits Scoticus, Traq., has a broad flattened 

 anterior part corresponding to the head and forepart of the 

 body, verj' bluntly rounded in front, and passing behind into 

 right and left angular flap-like projections, which are sharply 

 marked off" from the narrow tail, which is furnished with a 

 deeply cleft heterocercal caudal fin. Unless the flap-like lateral 

 projections are representatives of pectorals, no other fins are 

 present, neither do we find any teeth or jaws, nor any trace of 

 internal skeleton ; and it is only a few days since Mr. Tait^ 

 collector to the Geological Survey of Scotland; pointed out ta 

 me in a recently acquired specimen a right and left dark spot at 

 the outer margins of the head near the front, which spats may 

 indicate the position of the eyes.^ A previously unknown genus,. 

 Lanarkia, Traq., also occurred, in which the creature had the 

 very same form, but instead of having the skin clothed with 

 small shagreen like scales, possessed, in their place, minute 

 sharp conical hollow spines, without base and open below. 

 What we are to think of those two ancient forms, apparently 

 so primitive, and yet undoubtedly also to a great extent special- 

 ised, we shall presently discuss. 



Let us now lor a moment look at the genus Drepanaspis^ 

 Schlliter, from the Lower Devonian of Gmiinden in Westeri> 

 Germany.^ We have here a strange creature whose shape 

 entirely reminds us of that of Thelodus, having the same flat 

 broad anterior part, bluntly rounded in front, and angulated 

 behind, to which is appended a narrow tail ending in a hetero- 

 cercal caudal fin, which is, however, scarcely bilobate. But 

 here the dermal covering, instead of consisting of separate 

 scales or spinelets, shows a close carapace of hard bony plates, 

 of which two are especially large and prominent— the median 

 dorsal and the median ventral — other large ones being placed 

 around the margins, while the intervening space is occupied by 

 a mosaic of small polygonal pieces. The position of the mouth, 

 a transverse slit, is seen just at the anterior margin ; it is 

 bounded behind by a median mental or chin-plate, but no jaws 

 properly so called are visible, nor are there any teeth. Then 

 on each margin near the front of the head is a small round pit, 

 exactly in the position of the dark spot seen in some examples 

 of Thelodus, which, if not an orbit, must indicate the position 

 of some organ of sense. Again, the tail is covered with scales 

 after the manner of a " ganoid " fish, being rhombic on the 

 sides, but assuming the form of long deeply imbricating fulcra 

 on the dorsal and ventral margins. The position of the 

 branchial opening, or openings, has not yet been definitely- 

 ascertained. 



All these plates are closely covered with stellate tubercles, 

 and we cannot escape from the conclusion that they are formed' 

 by the fusion of small shagreen bodies like those of Thelodus^. 

 and united to bony matter developed in a deeper layer of the 

 skin. 



If the angular lateral flaps of Thelodus represent pectoral 

 fins, then we would have the exceedingly strange phenomena of 

 such structures becoming functionally useless by enclosure in 

 hard unyielding plates, though still influencing the general out- 

 line of the fish. Be that as it may, can we doubt that in 

 Drepauaspis we have a form derived by specialisation from a 

 Coelolepid ancestor ? 



ThK Drepauaspis throws likewise a much desired light on 

 the fragmentary Devonian remains known since Agassiz s time 

 as Psaviinosteus. These consist of large plates and fragments^ 

 of plates, composed of vaso-denline, and sculptured externally 

 by minute closely-set stellate tubercles, exactly resembling the 

 scales of some species of Thelodus. These tubercles are also 

 frequently arranged in small polygonal areas, reminding us 

 exactly of the small polygonal plates of Drepiinaspis, and, like 

 them, often having a specially large tubercle in the centre. 



1 R. H. Traquair, "Report on Fossil Fishes collected by the Geological 

 Survey in the Silurian Rocks of the South of Scotland," Trans. Roy. Soc. 

 Edin., vol. xxxix. 1899, pp. 827-864. 



2 1 am indebted to Sir A. Geikie, F.R.S., Director-General of the 

 Geological Survey, for permis-ion to make use of this and other facts dis-- 

 closed by Mr. Tait's work in the Lesmahagow Silurians during the present 

 summer. 



3 R. H. Traquair, Cfol. Ma^., Apiil 19 o. 



