2 BRITISH FOSSIL TRIGONLE. 



that any of the external ornaments are visible upon them ; and even under the most 

 favourable conditions the impressions of these ornaments are only faintly and insufficiently 

 shown, so that by means of these alone the external aspect even of a single species could 

 never be fully ascertained, and even when both the mould and test have been obtained it 

 is not in every instance that the mould can with certainty be discriminated from those of 

 other allied species. The practice of authors, therefore, who have described supposed new 

 species even partially, and have named them from internal moulds alone, is objectionable, 

 as tending to create doubt and hesitation in the minds of students, and encumbering the 

 list of species with things which for all practical purposes are little more than mere names. 

 Roemer, and afterwards Agassiz, sometimes unfortunately acted in this manner, and 

 D'Orbigny went even further in the same direction ; in his ' Prodrome ' are new names, 

 together with a few words to each, which are intended to indicate moulds only, that have 

 not even been figured. For the most part, therefore, internal moulds afford but little more 

 than so many proofs that Trigonise were buried in the stratum to which they refer, 

 and also that the species had apparently certain peculiarities of figure more or less 

 conspicuous. 



The figures of Trigonia given in the ' Petrefacta ' of Goldfuss are not very numerous, 

 but are almost invariably founded upon good specimens, and the execution of the plates 

 is equally satisfactory ; but the descriptions are only remarkable for a general vagueness 

 and brevity that so often is found to prevail in descriptions of fossils at the period when 

 they were written ; it is also certain that the geological positions mentioned are some- 

 times erroneous. The well-known memoir by Agassiz on the genus Trigonia contains 

 eleven plates, illustrative of forty-eight species ; it is much to be regretted that in many 

 instances these figures represent very indifferent specimens or fragments only, and that 

 these deficiencies are not compensated for by sufficiently copious or minute descriptions ; 

 they have been felt as a frequently recurring source of embarrassment, and more 

 especially when an author has adopted the name of one of these doubtful forms in a list 

 or description of species. It is, however, only just to remark that the work, upon the 

 whole, exhibits much of that discriminative faculty of mind which we should expect to 

 find in the production of so eminent a naturalist; the sectional divisions which he 

 proposed to establish are so far in accordance with nature, and are so well indicated by 

 his typical examples, that it would, perhaps, have been quite possible to follow out these 

 groups with little error throughout the entire genus, even if the author had given to each 

 of them simply a name without description ; in fact, the sectional descriptions are both 

 meagre and imperfect, and appear to have been intended as mere outline sketches, 

 susceptible of future enlargement or modification. D'Orbigny, in his 'Prodrome de 

 Paleontologie ' and in his ' Paleontologie Franchise,' has in some instances endeavoured 

 to correct errors in the identification of species by Agassiz, but not uniformly with 

 success, and apparently with only an imperfect knowledge of British Trigonia). Our 

 thanks are, however, due to him for having greatly extended our knowledge of the genus 



