18 W. S. MacLeay on the Structure and 



stance as the presence of eyes in the former and the want of 

 them in the latter^ may also induce us to fancy that similar 

 differences may have possibly occurred between certain male 

 and female Trilobiia, which from their prima facie difference 

 of form are now placed in distinct genera, although they may 

 have truly belonged to one and the same species. Serolis has 

 been generally considered to come near to Paradoxides ; but 

 as the former has got four well-developed antennae with crus- 

 taceous feet, and the latter none, I am inclined to believe the 

 relation between them to be one of analogy rather than of im- 

 mediate affinity. — Let us now turn to the Entomostraca. 



Dr. Buckland, following other authors, has compared the 

 Trilobites with the genera Limulus and Branchipus, With 

 the latter genus, however, they obviously have no immediate 

 affinity; although it maybe well, by reference to Branchipus, 

 to show that Crustacea can and actually do exist, with soft 

 membranaceous feet, such as Audouin and Brongniart sus- 

 pected, and Goldfuss has more lately asserted, to have been 

 the feet of Trilobites. When, nevertheless, 1 take into consi- 

 deration the perfect manner in which the soft body of an ani- 

 mal referred to me by Mr. Murchison, and by that gentleman 

 called Nereites Cambrensis, has lefl its impression in a slaty 

 rock, I confess 1 find it difficult to understand how the ves- 

 tiges of legs in a Trilobite (if such legs ever really existed) 

 should not be more evident than Goldfuss has represented 

 them in his plates. In short, I consider the question of feet 

 to remain still unsettled. At the same time I ought to remark, 

 that if the Trilobites were Crustacea, between Apus and Bo- 

 pyrus, a fact I conceive capable of demonstration, they must 

 have been in possession of subabdominal, laminar, oviferous, 

 appendages. Now, no traces of such appendages remain, 

 consequently we can easily understand how feet of a similar 

 membranaceous consistency may have disappeared in like 

 manner. I may here obsen^e, that Brongniart is certainly 

 wrong in imagining that the Ogygia Guettardi had oval ovi- 

 ferous bags appendent to the abdomen like Cyclops, for what 

 he considers to be such organs are more probably the mem- 

 branaceous margin of the abdomen, and, besides, Ogygia has 

 no immediate affinity to Cyclops, With reference to Limulus, 



