182 



A. S. PACKAED, JR., ON THE 



3, 1871), and the "unknown larva," p. 62. But when we compare the Eurypterida with 

 the normal larvae of Decapods, there is a general likeness, which, as Huxley has clearly 

 pointed out, is of peculiar significance. This is seen in the small cephalothorax, with the 

 compound eyes sunken in the head, as in some zoe?e, aud the long, narrow, equally-jointed 

 abdomen, with its spine-like or spatulate telson. Here we have, if we are not mistaken, 

 prophetic forms, which peopled the palteozoic seas, and announced the coming of a higher 

 type of crustacean life. The gigantic Eurypterida are to the Decapoda of the present 

 times what the Pythonomorpha and Ichthyosauria and Plesiosauria are to their modern rep- 

 resentatives ; and wdiether or not the Decapoda be the direct lineal descendants of the 

 Eurypteridiie, they are, at least when in the early stages of growth, heirs of some of their 

 peculiar features, though they may claim a distinct line of descent from the ancestors of a 

 hypothetical nauplius, with new and higher characters superadded. 



Comparison of Limulus with the Trilohites. Let us now compare the larva of Limulus 

 with those of the Trilobites. Barrande, in his great work on the Silurian Fauna of Bo- 

 hemia,^ has been the first to describe the recently hatched larvae of certain Trilobites, at least 

 before the young had moulted, and has presented us with an array of facts, that are of the 

 greatest use in our comparison. 



A glance at our figures 12 and 32, will at once suggest the form of the Trilobites, as also 

 of the Xiphosurian genera Bellinurus, Prestwichia, and Euproops, (see cuts 30, 31, Prest- 



wichia rotundatiis, Prestw., 

 Euproops Dance, Meek and 

 Worthen^) from the Carbonif- 

 erous formation. The latter 

 are evidently forms which may 

 be compared with the larva of 

 Limulus, from the prominence 

 of the cardiac lobe, the dis- 

 tinctness of the abdominal seg- 

 ments, and the short spine, 

 while the form of the carapace 

 is stiU more trilobitic in the 

 posterior prolongations of the 

 outer edge. These forms are 

 alluded to here as indicating in Carboniferous times the existence of intermediate types 

 between the Merostomata and Trilobites. 



The resemblance of the ripe embryo, and in a less degree, the larva of Limulus, to the Tri- 

 lobites is seen in the very distinct trilobate character of the body — the cardiac lobe being 

 very prominent, — and in the distinct segments of the abdomen. The resemblance is com- 

 pleted by the absence of the caudal spine of the adult Limulus. In comparing the larval 

 forms of Limulus with those of the Trilobites, as described and figured by Bari'ande, we are 

 struck by the close correspondence between them. We have endeavored to show that the 

 larva of Limulus is to be compared with the zoea of the Malacostraca. We should not intro- 



' Systeme Silurien du Centre fie la Boheme. Par Joachim ^ I am indebted to the Illinois Geological Sin-vey, through 



Barrande. 1'" Partie. Recherehes Paleontologiques. Vol. Professor Worthen, for the use of these cuts. 

 1. Crustaces ; Trilobites. 1852. 



Fig. 30. 



Ficf. 31. 



