60 INTRODUCTION. 



to Prof. Quciistedt and some others, this last group would also take place among the 

 Terehratida, an assumption surely in opposition to zoological law. 



The great difficulty seems how to estabhsh a definite and applicable arrangement ; and 

 this question will probably continue unsolved, until our knowledge of the internal struc- 

 ture of the many at present ambiguous forms has been cleared up, which fortunate 

 circumstances, time, and persevering researches can alone accomplish.^ 



The many years I have devoted to the study of the species composing this class, has 

 tempted me to propose certain amendments and changes in the schemes already published 

 by Prof. King, M. d'Orbigny, and others ; and I do so in the hope that any steps, 

 hoioever small, towards the future settlement of a so-much controverted question, may be 

 considered worthy of the attention of those who feel interested in the subject. 



^ A celebrated French conchologist stated some years ago, that Palaeontologists could never hope to 

 arrive at a knowledge of the interior of all the species ; but since that period the numberless discoveries and 

 well-conducted researches make us believe, on the contrary, that ere long we may become acquainted with 

 the most delicate internal details of all those species which have hitherto resisted our endeavours. And no 

 better example can be selected of what patience and diligent exertions can achieve, than the splendid work 

 recently published on the 'Silurian Fossils of Bohemia,' by that eminent and justly celebrated geologist 

 and palaeontologist, M. Barraude ; the author mentions, among other examples, that to obtain a perfect 

 example of one of the most common Trilobides {Dalmanites socialis), it required a number of years of the 

 most assiduous researches. Among the Brachiopoda, we may likewise quote Stringocephalus and Uncites, 

 whose internal arrangement baffled all researches for a long period. 



