38 THE CRAG POLYZOA. 



Dr. Johnston, and although it does not appear to have been used by any Continental 

 writer, has been so generally employed by English naturalists, and is so appropriate and 

 free from ambiguity, as to claim general approbation. 



The very large number of species, both recent and extinct, included in the Genus 

 Lepralia, renders it one of the greatest importance. It appears to date from the 

 Cretaceous period, and species belonging to it abound in the tertiary beds, as well as 

 at the present epoch. As might be expected in a group of related forms enjoying so 

 wide a distribution in time, they are found to enjoy an equal existence in space. Of 

 the numerous species some are found in all parts of the globe, from the Arctic to the 

 Antarctic regions, many being apparently cosmopolite. This capacity for general diffusion 

 appears to be conjoined with a power of adaptation to different circumstances, perhaps 

 greater than resides in any other genus of Polyzoa ; and thus arises a disposition to varia- 

 tion which adds very considerably to the difficulty of discriminating species. It is not 

 improbable, also, that owing to this peculiarity, some of the presumed extinct forms might 

 be traced by careful research into connexion with those still living. For the same reason, 

 also, it is highly probable that continued study may show that many species now regarded 

 as distinct are in reality only varieties. But notwithstanding this, there can be no doubt 

 of the existence of a vast number of truly distinct forms, whose similitude renders it indis- 

 pensably requisite, for the convenient determination of any particular species, to subdivide 

 the genus into minor groups, which, however, must be regarded simply as artificial, and 

 contrived solely for the purpose of convenient reference. 



The following synopsis is pretty nearly the same in form as that employed in the British 

 Museum Catalogue, Part ii, p. 03. 



