No. 634] PHYLOGENY OF THE ARTHROPODA 407 



If dorsal tests only be considered, one can pick out an 

 excellent series showing gradations from a trilobite into 

 Limulus. Thus, there are in the Middle and Upper Cam- 

 brian the Aglaspidae, with Limulus-like head, trilobite-liko 

 free thoracic segments, and Limulus-like telson. Follow- 

 ing the history of the group through the Paleozoic, there 

 is, in the Silurian, Neolimulus with a head which is surely 

 that of Limulus but which has vestigial facial sutures, 

 and free thoracic segments are present. By Devonian 

 times, the thorax had begun to fuse into a shield, although 

 some of the Pennsylvanian species retained a few free 

 segments at the anterior end of the thorax. This series 

 is so convincing that one must believe that the Xiphosura 

 developed from the trilobites, but a study of the append- 

 ages shows that there are greater differences between 

 those of a trilobite and Limulus than between those of a 

 trilobite and one of the highest Crustacea. 



The greatest difference is in the complete lack, at any 

 stage of development, of the antennules, from which it 

 follows that the anterior shield in the Xiphosura is a 

 cephalothorax in which at least seven segments are in- 

 corporated. It is, of course, entirely possible that in one 

 line of evolution of the trilobites the antennules were lost 

 and the antennae developed as chelicerae, but if the test be 

 applied to the Aglaspidae, the results are at variance with 

 the expectation. Walcott has found Aglaspidae with ap- 

 pendages in the Middle Cambrian, and they seem to have 

 five pairs of appendages on the cephalon, two pairs of 

 which are elongate, multi-segmented, tactile antennae, and 

 biramous appendages are present on the thorax. 4 These 

 animals were still Crustacea, and the development of 

 elongate antennae instead of chelicerae shows that they 

 were not tending in the direction of the Xiphosura. It is 

 possible, however, that when the appendages of more 

 Aglaspidae are known, it will be found that some of them 

 showed a tendency to lose the antennnles and develop 

 chelicerae. 



* Smithsonian Miscl. Coll., Vol. 57, No. 6, 1912. 



