TRANSACTIONS OF SECTION D. 771 



Marsipobranclis may now be said to be given up. They are now accepted by the 

 most reliable authorities as appertaining to Invertebrata such as Aunelides and 

 Gephyrea. 



More recently, however, Rohon * has described from the Lower Silurian of the 

 neijjhbourhood of St. Petersburg small teeth {Palaiodus and ArcJiodus) 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 supposed to be of Lower Silurian or 

 Ordovician age.'- But doubts have been thrown on their age, and the fossils them- 

 selves, -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 Ccelolepidaj, having a flat or sculptured crown, below 

 which is a constricted ' neck,' and then a base usually 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 {0?ickus), they were 

 usually considered as appertaining, with the latter, to small Cestraciont sharks. 

 The genera Thelofhis, Ccalole^nK, and others were 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 discovery 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.^ 

 Thelodus scoticiis, Traq., has a broad flattened anterior part correspondinn- to the 

 head and forepart of the body, very bluntly rounded in front, and passino- behind 

 into right and left angular flap-like projections, which are sharply marked olf 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 to me in a recently acquired specimen a right and 

 left dark spot at the outer margins of the head near the front, which spots 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 havinf 

 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 

 gi'eat extent specialised, we shall presently discuss. 



Let us now for a moment look at the genus Drepanaspis, Scbliiter, from the 

 Lower Devonian of Gmiinden in Western Germany.* We have here a strange 



' ' Ueber untersilurische Fische,' 3Ielanges Gcol. ct Paleont. vol. i. (St. Petersburg, 

 1889). pp. 0-14. 



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



' R. H. Traquair, ' Report on Fossil Fishes collected by the Geological Survey in 

 the Sihirian Rocks of the South of Scotland,' Trans. Roy. Soc. Edin., vol. ssxix. 

 1899, pp. S27-8Gi. A. specimen of Thelodus had, however, been found by Mr. James 

 Young, of Lesmahagow, before the Geological Survey came on the scene. 



* I am indebted to Sir A. Geikie, F.R.S., Director-General of the Geological 

 Survey, for permission to make use of this and other facts disclosed by Mr. Taifs 

 work in the Lesmahagow Silurians during the present summer. 



=■ R. H. Tra.^uair, Geol. Mag., April 1900. 



