Notices of Memoirs — Dr. R. H. Tmquair's Address. 465 



Silurian of the neighbourhood of St. Petersburg small teeth (Palaodus 

 and Archodus) 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 

 Walcott, 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 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 specialization 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 Ccelolepidae, 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. 



The genera Thelodus, Coslolepis, and others were founded on these 

 dermal bodies, and complete specimens of Thelodus have now been 

 found in the Upper Silurian rocks of the South of Scotland, from 

 which it is evident that the fish, though somewhat shark-like, can 

 hardly be reckoned as a true Selachian.- Thelodus Scoticus, Traq., 

 has a broad flattened anterior part corresponding to the head and 

 forepart of the body, very 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. Under the flap-like lateral 

 projections ai'e 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, LanarJcia, Traq., also occurs in 

 which the creature has the very same form, but instead of having 

 the skin clothed with small shagreen-like scales, possesses 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 specialized, 

 we shall presently discuss. 



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

 from the Lower Devonian of Gmiinden in Western Germany. We 



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



2 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. 



3 R. H. Traquair: Geol. Mag., April, 1900. 



DECADE IV. — VOL. Til. — NO. X. 80 



