CERATODUS. 27 



Oolitic matrix, and probably derived from the same locality. The remaining British 

 species come from the Rhsetic beds of the Severn. The bulk of the great collections 

 made by Higgins and others are assembled in the Bristol Museum, but useful series are 

 possessed by the British Museum and the Museum of Practical Geology. 



Among the 350 fossil Ceratodus teeth of the Bristol Museum a number of examples 

 seem to be recognised at the first glance as specifically distinct. Some are extremely 

 broad, others narrow ; many have the first denticle much elevated, in others all the 

 denticles are more or less elevated, a few are nearly plane. Strong pitting is usually 

 apparent, but in occasional examples the surface is smooth to the eye, and presents a 

 minute punctation only when closely examined. The curvature of the grinding surface, 

 the thickness of the dentinal layer, and the length of the transverse ridges vary just as 

 conspicuously. It is not surprising that Agassiz, dealing with a few only of these 

 variable forms, should have found nearly every one specifically distinct. 1 Where specimens 

 are few the naturalist is driven to use specific characters of whose systematic value he is 

 ignorant. Even now that the collections have increased so greatly I have found it no 

 easy task to discover the natural gaps in the series, and after nearly two years' study I 

 was prepared to define five or six species where I now see only two. Still earlier, when 

 entering upon the examination of the Bristol collection, the limits of species seemed 

 impossible to be traced. Nearly every tooth had features of its own ; there was no rule 

 for distinguishing palatal from mandibular teeth, nothing known about succession, nothing 

 about the effect of age upon the dentition. The only means of clearing up these difficul- 

 ties was to put like things together, and find out by perpetual comparison what grada- 

 tions of structure existed. In the progress of this tedious but necessary work, the 

 Ceratodonts from the Deccan came to remove one perplexity, and henceforth it was 

 possible to distinguish palatal from mandibular teeth. 2 The question of succession also 

 was disposed of, though not so conclusively as one could wish. 3 After many abortive 

 attempts, which revealed the untrustworthiness of nearly every character hitherto relied 

 on, it was found possible to arrange the almost infinitely varied forms — large and small, 

 long and short, broad and narrow, of all shapes, and of different thickness — in two series, 

 each free from disturbing intervals. In the absence of a rich supply of specimens this 

 reduction could not have been made, or at best must have been made on unsatisfactory 

 grounds — an argument for the retention in public collections of what to an uninterested 

 visitor are mere duplicates. 



Eor the two species which thus emerged from a perplexed multitude it was necessary 

 to select names from those already proposed, or to get rid of confusion by giving new 

 ones. Agassiz's names, alius, emarginatus, &c, are too descriptive, and would prove mis- 

 leading if used for the new aggregate. It would also promote confusion to select at 



1 Agassiz speaks of more than one example in the cases of alius and planus alone. 

 3 Supra, p. 21 ; Miall, ' Paheontologia Indica ' (1877). 

 8 Supra, p. 26. 



