FOSSIL FISHES. 617 



The fossils are unfortunately not very well preserved, being, as is usual 

 in this formation, heavily pyritized. But to the palaeontologist they are 

 nevertheless invaluable. Thus far we have known nothing of the form 

 and appearance of these early sharks. Containing, as do all the rest of 

 their family, a skeleton that was almost entirely cartilaginous, they left 

 few fossil bones or plates to carry down to us an idea of what they were. 

 Their disjointed teeth which, being attached to the skin of a jaw for the 

 most part cartilaginous, were set free by its decay and strewn over the 

 sea- bottom, together with the strong and often highly ornamented spines 

 which fronted the dorsal and other fins, have hitherto been almost the 

 only relics of the shark-life of the older seas. On these teeth and spines 

 genera. and species have been founded for want of better data, and doubt- 

 less in not a few cases the temporary nomenclature thus established will 

 be found largely synonymous. Not a few of these genera and species are 

 based on a single tooth, and when we consider the variety of form and 

 pattern of the teeth in the mouth of a single shark we can fully under- 

 stand how one fish may be bearing several names. This is unavoidable 

 in the present state of our knowledge, but cannot lead to serious error 

 except in those who are unacquainted with the limits of discovery. The 

 palaeontologist will not be deceived, because he well knows that these 

 terms are merely the names of teeth or spines and not of fishes, and he is 

 awaiting the time when their connecting links shall be found. 



These few words of explanation will serve to show the value of the 

 recent discoveries in the Black Shale of Ohio. Though in consequence of 

 their want of distinctness we are unable to characterize the species as 

 fully as is desirable, yet we are able for the first time to form a concep- 

 tion of the general form and outline of these primaeval elasmobranchs 

 and to recognize in them many of the features that mark their descend- 

 ants of to-day. 



The specimen described and figured by Dr. Newberry was found by 

 Mr. Fyler. A second was discovered by the Rev. W. Kepler and a third 

 by Dr. Clark. The last was figured under the name of Cladodus Fyleri, 

 but no description was given and there is apparently very slight ground 

 for its separation from the former. No teeth are visible. 



Since the Monograph was issued several other specimens of these 

 sharks have been found by Dr. Clark, indicating the existence of other 

 species, one of which will be described below. 



Considering the nature and date of these fossils we are justified in 

 drawing the inference that the sharks are among the oldest ichthyic 

 inhabitants of our globe. They shared the Devonian seas of Ohio with 

 the Placoderms and their teeth are found yet deeper in the Corniferous 

 Limestone. Shagreen, indicating shark-life, occurs in the English Ludlow 

 or Upper Silurian rocks among almost the oldest fish-remains known in 

 the Old World and no vertebrate remains of undoubted authenticity are 

 yet known below the Upper Silurian strata. We must therefore date the 



