BURMEISTER ON TRILOBITES. 131 



In considering this latter important question — that of classifi- 

 cation, there seems no doubt as to the fact that the Trilobites are 

 Crustaceans, although whereabouts in that group they are to be 

 placed is not so clear. Dr. Burmeister subdivides the Crustacea 

 into two divisions — the Malacostraca, which are well known, and 

 the Ostracodermata, these latter having compound eyes with 

 simple horny membranes, all of them passing through various 

 metamorphoses, and exhibiting more striking contrasts at different 

 periods than the animals of the other division. These Ostracoder- 

 mata consist of two groups, each comprising three families, the 

 animals of the first group not having a distinct head with true 

 antennas and eyes, while those of the second are distinguished by 

 the presence of very large and often enormous eyes and antennae, 

 which are most fully developed when the eyes are smaller. The 

 two groups differ also in the nature of the metamorphosis which 

 the species undergo. A table is given of the principal characters 

 of these groups, and the author proceeds to consider to which of 

 them the Trilobites are most nearly allied. 



In the first place, he argues that they cannot belong to the 

 Malacostraca, and that therefore all the apparent analogies with 

 Isopoda at once cease to possess any value. * This conclusion he 

 grounds upon the fact that they neither have true facetted eyes 

 nor an ordinarily formed thorax, nor the constant number of from 

 five to seven thoracic rings. The want also of large antennae, of 

 the broad shield-shaped head, and of visible articulated feet, and 

 the unequal number of abdominal rings, all oppose such a view. 



The resemblance of the Trilobites to Limulus is another analogy 

 which Dr. Burmeister does not admit. The absence of detached 

 head and thorax in this genus renders it impossible that there 

 should be a close aflinity ; while the structure of the feet here also 

 is dissimilar, these being perfectly well preserved in the Limuli of 

 the Jura formation, so that they must have been covered with a 

 hard or horny coat. For these and other reasons, the author con- 

 siders Limulus to be still more widely removed from the ancient 

 Trilobites than the Isopoda are. 



Since then the Trilobites, from their analogies, seem to belong to 

 the Ostracodermata rather than the Malacostraca, we have next to 

 consider to which of the two principal groups of Ostracodermata 

 they must be referred. The presence of large eyes sufficiently deter- 

 mines this question, and they fall therefore among the Aspidostraca 

 or Entomostraca, with all the principal characters of which groups 

 they are shown to agree (p. 40.). It appears, also, on more minute 

 examination and comparison, that of the three families of this 

 group, the Trilobites most nearly resemble the members of the 

 Phyllopoda. 



* The singular species Serolis paradoxa is referred by Burmeister to the 

 Phyllopoda, the second family of the second division, as alluded to above. He 

 denies the close affinity of this species to the Trilobites. 



K 2 



