422 



HISTORICAL GEOLOGY. 



hypostome. The eyes may be very large, as in Dalmanites (Fig. 386), Phacops, and Asa- 

 phus (Fig. 689), or small, as in Homalonotus ; or not at all projecting, as in Trinucleus 

 (Fig. 692) ; and may also differ in position in different genera. 



387-391. 



390 



Triarthrus Bec'KII. — Figs. 387, 388, specimens with antennae and portions of cephalic and thoracic ap- 

 pendages (X 2) ; 3S9, portion of antennae (x 10) ; 390, posterior half, with remains of feet (x 2) ; 391 a, 

 one of the jointed appendages (x 6) ; 391, one of the feet. Matthew. 



Specimens of Trilobites are almost always without appendages of any kind. Evi- 

 dence of pairs of slender limbs extending the whole length of the body were first observed 

 in a specimen of Asaphus platycephalus, by Billings, in 1870 ; and later, in 1883, in 

 another American species, A. megistos, by Mickleborough. New proof was announced 

 by Walcott, in 1876, 1877, and 1881, from siloings of some hundreds of specimens of 

 a species of each of the genera Calymene and Ceraurus; who reached the conclusion 

 that there were four pairs of slender appendages to the head-portion, and a series along 

 the whole under surface to the extremity of the pygidium or abdomen. He also obtained 

 evidence that the thoracic legs had at bases a branch (epipodite), and that they carried 

 also an appendage in the form of slender filaments, some of which were spiral, which he 

 described as probably branchial. Mr. Walcott also gives figures of what he regards as 

 the fossil ova of the Trilobites. 



These results have been iu the main confirmed and made more definite from specimens 

 of Triarthrus Beckii, found by W. S. Valiant, and described, in 1893, by W. D. Matthew, 

 some of which are represented in Figs. 387-391, from Matthew's paper. In addition to the 

 existence of legs, the specimens figured show that Trilobites had slender antennse of the 

 first pair (Figs. 387, 388), consisting of short joints (Fig. 389); and that the slender, 

 bifid, jointed feet were, in part at least, natatory organs, probably, by plumose setae (as is 

 indicated in Fig. 388 and others). The presence of a second pair of antennse is probable, 

 but none is indicated. The specimens were from a thin layer in the Utica shale near 

 Rome, Oneida County, New York. 



Later investigations of specimens from the same locality, by C. E. Beecher (1893, 1894) 

 have ascertained that the abdominal appendages are branchial, as in modern Isopods ; 

 he has also made out the precise form and other characters of the thoracic limbs, show- 

 ing that each consisted of a seven-jointed leg, and a long natatory appendage. (See page 

 512 for figures.) 



The following table exhibits the homologies, as regards segments and their appendages, 

 of different types of Crustaceans. indicates the absence of a segment, and the Roman 

 numerals above, the normal number of the segments in the cephalothorax and abdomen. 



