48(3 A. J. JUKES-BROWN e's SUPPLEMENTARY 



of the same intrinsic value as those which are founded upon a 

 number of perfect examples ; neither can they be of the same use 

 or importance to the comparative palaeontologist. We are led, 

 therefore, directly to the conclusion that some species are of more 

 palaeontological value than others ; and it may be worth while con- 

 sidering a little more fully the other circumstances by which this 

 value is modified or increased, especially as I am not aware that 

 any one has recently discussed the conditions under which new 

 species may be constituted, or given any directions to those intending 

 to name and describe such forms as may appear to have been pre- 

 viously unnoticed. 



The importance of a proposed new species must greatly depend, 

 I think, upon the following circumstances : — 



1. The number of specimens examined (whether many or few). 



2. The number of localities w r here it has been found. 



3. The extent to which the original shell or test has been 

 preserved. 



4. The amount of difference between it and nearly allied species 

 of the same genus. 



In the first place it is evident that a species founded on a single 

 specimen, however well preserved, has not the same value as one 

 the description of which has been drawn from an examination of 

 many individuals. Much, however, will depend upon the character 

 of the fossil ; and supposing all the other conditions to be very 

 favourable, if it is in a good state of preservation, retaining all 

 its parts, and if it be markedly different from any other species, 

 so that it is unlikely to be merely a monstrosity or local variety, 

 then its intrinsic value is much enhanced ; but at the same time it 

 remains of little use when considered as a member of the fauna 

 to which it is added ; for, being found only at one locality, it cannot 

 be utilized in a comparison of one fauna with another, but must 

 always be omitted from such calculations. 



Again, new species which have been described, from single 

 imperfect specimens in the state of casts, and even those founded 

 on casts which are abundant and perfect so far as they go, are, as a 

 rule, very unsatisfactory; for it is only in exceptional cases that the 

 cast indicates the form and ornamentation of the shell with which 

 it was covered. In cases where the cast does give such evidence 

 its description is perhaps of some value ; but where the casts are 

 nearly smooth while the test was probably ornamented, as in Bivalve 

 shells, Echinoderms, and many Gasteropods, it is worse than useless 

 to describe them : for it is often impossible to compare them properly 

 with specimens from other deposits which retain the shell, and the 

 result is an unnecessary multiplication of specific terms : it is far 

 easier to give a specific name to such casts than it is to disprove 

 their title to the same. 



Again, it is exceedingly difficult, even where specimens are 

 abundant and their state of preservation good, to be quite sure that 

 the form has never been elsewhere described. If not known in the 

 British Isles, it may have been figured and described on the Con- 



