186 A. S. PACKARD, JR., ON THE 



Bd. 6, Heft. 1, p. 131, 1870) he remarks : "The external structure teaches us before all, 

 that we need seek for no swimmers in the Trilobites. Their body is broad and flat, very 

 generally they are broadest before, at the head, a fact also certainly adverse to a rapid for- 

 ward movement. We have already seen that a limitation of the power of swimming to the 

 pleon (abdomen) decreases the longitudinal extension of the body; in the Trilobites the pleon, 

 on the other hand, becomes a powerful, ponderous anal shield, the so-called pygidium, which 

 either consists of numerous segments united together, or we find a whole series of smaller 

 rings, but which do not in the least differ from the preceding segments of the middle 

 body. It is also to be considered that the Trilobites chiefly lived at the bottom of the sea. 

 Therefore they needed extremities by means of which they could walk. These extremi- 

 ties could not have been soft and yielding, since the Trilobites were truly heavy organisms. 

 According to my opinion, we must conceive their extremities to have been lilie those of 

 Limulus, but not longer than the carapace, or we should have detected traces of them. 

 These extremities, as locomotive organs, were attached only to the cephalic shield — the 

 rings of the body [posteriorly] carried most truly flat, leaf-like appendages, on whose inner 

 side again, as in Limulus, numerous thin leaves formed the gills. I cannot consider that 

 the Trilobites possessed extremities lilve the Phyllopods now living — with the power that 

 they possess of rolling up their bodies, it seems the better opinion to compare their gills 

 and the extremities carrying them to the gills of the existing Limulus." In considering 

 the fact that no specimens of Trilobites have revealed any limbs from a dorsal view, it 

 should be remembered that in none of the fossil Bellinuri, and even the Mesozoic Limuli, 

 are there any indications of limbs projecting beyond the carapace ; in death they are 

 gathered up beneath the body, and those of the Trilobites, if they existed, probably were 

 not long enough, as Dohrn says, to reach to the edge of the body when extended. 



In view of the conflict of opinions as to the nature of the limbs of the Trilobites, it is to 

 be hoped that the matter will not be suffered to rest here by palaeontologists, even if the 

 most unique and valuable specimens have to be sacrificed in making the requisite ob- 

 servations. 



Adding to the homologies set forth by Mr. Billings, the trilobate character of the body in 

 the young Limulus, the position and structure of the compound eyes, and the probability 

 that, as Billings suggests, the abdomen of the Trilobites carried several pairs of phyllopodous 

 or gill-like swimming feet, and we feel warranted from these, and the fact of their similar- 

 ity in the larval forms, in placing the Merostomata and Trilobites near together in the sub- 

 class of Branchiopoda. 



Through Barrande we learn that Beyrich has discovered the alimentary canal of Trinii- 

 cleus Golclfussi Barr. The former states, " several specimens of our collections have also 

 shown to us this canal which can be followed from the middle of the glabella up to the 

 extremity of the axis, towards the posterior border of the pygidium. M. Beyrich has well 

 observed that the canal in question is not found immediately under the test, but in the 

 interior of the median lobe, of which it seemed to have formed the axis." 



The internal organization and the habits of Limulus throw much light on the proba- 

 ble anatomy and habits of the Trilobites. The correspondence in the cardiac region of 

 the two groups shows that their hearts and circulation were similar. The position of the 

 eyes shows that the Trilobites probably had long and slender optic nerves, and indicates 

 a general similarity in the nervous system. The genital organs of the Trilobites were 



