366 CRUSTACEA ENTOMOSTRACA . 



and primitive members of the Crustacean series, it is most 

 undesirable to press the evidence for more than it is worth ; 

 and in the foregoing account of the appendages an attempt has 

 been made to show where, as appears from the published 

 descriptions, the ground is still insecure. It seems certain 

 that they possessed only one pair of antennae, and, taking 

 into account the indications of segmentation in the glabella 

 of the larva and of the adult in addition to the evidence 

 afforded by the appendages, the existence of four other cephalic 

 limbs, provided with gnathobases, appears probable. The 

 establishment of these results would confirm in the most 

 striking manner the conclusions which have been arrived at from 

 the study of the existing fauna on the nature of the second an- 

 tennae of Crustacea namely that they are postoral appendages, 

 which have in recent forms become preoral. 



Besides the characters of the second cephalic limbs and the 

 indication of segmentation of the glabella into five segments, 

 we may recognize as primitive features the presence of limbs 

 on all the segments except the telson, the varying and usually 

 large number of the body segments and the small degree of 

 specialization in the series of appendages, though how far 

 this applies to the head appendages must remain for the pre- 

 sent undecided. 



In the two last features Trilobites resemble the Phyllopods 

 among the Branchiopoda, to which group Burmeister had pointed 

 out their resemblance before the discovery of the appendages. 

 It is also shown in the large hypostoma and the gnathobases 

 of the post-cephalic limbs ; and the absence of a carapace finds 

 a parallel in Branchipus and its allies. On the other hand the 

 deeply cloven character of the limbs removes the Trilobites 

 from the immediate neighbourhood of Phyllopods, as also from 

 all other recent forms. 



Until the appendages were discovered Trilobites were 

 usually classed with Limulus and its allies, on account of the 

 shape of the head shield, the position of the eyes and the 

 so-called " trilobite stage ' : in the development of Limulus. 

 This association is indeed still retained by many authors, 

 but the indication of five pairs of cephalic appendages, as in 

 the Crustacea, not seven as in the cephalo -thorax of the 

 Merostomata, and the evidence which we now have on the 



