IV. 



The Problem of the Primaeval Sharks. 



IF the earliest true fish could be found, it would almost certainly fall 

 within the sub-class Elasmobranchii. The progress of discovery 

 of the primaeval sharks in Palaeozoic rocks is thus followed by ichthy- 

 ologists with increasing interest. The remarkable fossils described 

 within the last few years have in many respects revealed the most 

 generalised type of fish of which we can conceive ; and at the same 

 time it has become apparent that there was great variety among these 

 old genera, several of their parts attaining an excessive development 

 in different directions. 



One imperfectly known group, for instance, in which the dermal 

 armour has the microscopical structure of that of Elasmobranchs, is 

 gradually becoming recognised as a characteristic type of the Upper 

 Devonian and Lower Carboniferous periods. It is remarkable for 

 the excessive development of this armour, which assumes the form of 

 great plates and more or less triangular spines, the latter attached to 

 the skin only at their base. The Devonian examples are described 

 chiefly under the name of Psammostens, while most' of those of Car- 

 boniferous age are termed Or acanthus, Gyracanthus, known only by 

 paired spines, is also probably a closely related genus. 



Psammostens has just been discovered in the Upper Old Red Sand- 

 stone near Elgin, by Mr. William Taylor, of Llanbryde, and new 

 interest has thus been aroused in the fish (6). Hitherto it has been 

 found only in corresponding strata in Russia and Spitzbergen, and in 

 boulders scattered over the North German plain (8). The largest 

 collection of its remains is preserved in the Geological Museum of the 

 University of Dorpat ; but even here there is not enough evidence to 

 show how the armour of the fish was arranged. A brief notice of 

 the various plates and spines of Psammostens, which the present writer 

 has had the privilege of examining in that museum, may, however, 

 now prove opportune. 



The most interesting fragment of armour from the Upper Old 

 Red Sandstone of Neuhausen, Livonia (where an Asterolepis much like 

 A. maxima occurs), is shown of one-quarter the natural size in the 

 accompanying Fig. i. The original specimen at Dorpat is much 

 abraded, but it still bears traces of having been ornamented externally 

 in the manner characteristic of Psammostens paradoxus ; and there is a 

 second specimen in the same collection having the ornament well- 



