i84 OUR COMMON BRITISH FOSSILS. 



of the development of the lobster, all the above 

 changes noted by Barrande in the Trilobites occurred 

 before the animal had attained a tenth part of its full 

 size. In Lyell's " Manual of Geology " the student 

 will find engravings of the Trimtcleus in three stages, 

 each of which appears specifically distinct from the 

 other. Another skilled observer of the Trilobites was 

 Burmeister, who believed that all of them underwent 

 metamorphoses. Recently the larva or young of a 

 beautiful and highly developed Trilobite {Conocoryphe) 

 has been found in this country. In the case of fossils 

 less care has been taken than with living animals, and, 

 in many instances, some of those who have christened 

 species were geologists rather than naturalists. The 

 slightest difTerences have been sufficient to warrant a 

 new specific name, and thus it is more than possible 

 that the various stages in the life-history of one and 

 the same species may be illustrating our manuals as 

 distinct genera and species ! Even with regard to sex 

 in adult individuals,little or nothing is known ; although 

 among nearly all the Crustacea these differ so ex- 

 tremely. Owen remarks that the difference in the 

 head plate and the terminal spines of the tail in the 

 two so-called species named Asaphus caiidatus and 

 AsapJms lojigicmidatiLs, may only be due to difference 

 of sex ; the inference, therefore, is that these two 

 species represent the male and female of only one. 



The earliest Trilobites, such as the eyeless Agnostits, 

 are usually the simplest in structure, so that these 



