235 



CHAPTER XI. 



AND HELIOPORID/E. 



THECID^E. 



THE group of the Thecidce, E. and H., comprises only the 

 anomalous Silurian genus Thecia, E. and H., the type of which 

 is the Thecia Swindernana of Goldfuss. It is important to 

 bear this fact in mind, as I shall found the following descrip- 

 tion of the group entirely upon a most careful macroscopic and 

 microscopic investigation into the structure of the type-species 

 of the genus, of which I possess a large number of authen- 

 tic examples, derived from the Wenlock Limestone of both 

 Britain and Sweden. I mention this the more particularly 

 here, because it is needless to say that the diagnosis of a genus 

 must in all cases be ultimately decided by the structural char- 

 acters of the original type of that genus, and cannot be based 

 upon forms which any given observer may believe, - perhaps 

 upon insufficient grounds, to be congeneric with that type. 

 The genus now under consideration illustrates this point in 

 an especial manner, for the last, and in all respects the most 

 circumstantial, account of the genus Thecia, E. and H., which 

 has yet been published (Foss. Cor. of Michigan, p. 65, 1876), 

 ascribes to the genus characters such as do not exist in the 

 type-species, and ignores other fundamental features which are 

 really present in the latter ; and I do not think that so accom- 

 plished an observer as Dr Rominger could have arrived at 



