C. D. Walcott — Appendages of TrUohites. 251 



locality ; that both occur within the Ordovician ; and that the 

 stratigraphic position of the bed at Rome is between six and seven 

 hundred feet above that at Trenton Falls. ^ 



EXPLANATION OF PLATE VIII. 



Fig. 1. — Triarthrus Becki ( x 3). Outline of Carapace, with appendages represented 

 as they occur on several specimens, their relative position being 

 retained. 



a, a, a, a. Endopodites of limbs showing variation in joints. 



b, b. Plumose portion of exopodite. 



c, c. The outer or supporting portion of the setae or fimbriae of b, h. 



d, Limbs extending from beneath the pygidium, showing large proximal 



joints. Those of the left side are imperfectly preserved. 



e, Antenna extending back nearly to the postero-lateral margin of the 



hypostoma. 

 /. One of the cephalic limbs. The basal joint may be broken away on 



the inner side. 

 g. Cephalic limb. 

 Fig. 2 (x7). — Limbs attached to the under surface of an individual preserving 13 

 thoracic segments and the pygidium. The limbs {a to k) on the left 

 side are mainly in place. A fracture cuts out one limb between 

 g and h. 

 a to g. Limbs preserving traces of the enlarged proximal joints. 

 b, d. Limbs preserving the two joints of the protopodite and two of the 



' large proximal joints. 

 ?, in, 0. Exopodites, showing under or side views. 

 n. Enlargement of fimbriae of m. 

 r, s. Distal joints of endopodites of right side. 

 q. Portion of an exopodite showing its inner support. 

 Fig. 3. — Limbs occurring on the under side of an individual of 14 thoracic segments. 

 a, b, c, d. Limbs with flattened, enlarged proximal, and slender distal 



joints. 

 a. Limb preserving large joints of protopodite (^j), four enlarged proximal 

 joints and three slender distal joints. At x the point of attachment 

 of an exopodite is shown, and in the specimen it looks as though 

 f had been broken away from x. 

 Fig. 4. — Restoration of the thoracic limbs of the fifth segment anterior to the 

 pygidium. 

 en. Endopodite. p. protopodite. a. four proximal swimming joints. 



b. three distal joints. 

 ex. Exopodite, attached to same joint of the protopodite as the endopodite. 

 Fig. 5. — Eestoration of the thoracic limbs of the fourth thoracic segment posterior 

 to the head. 

 en. Endopodite. ex. exopodite. 

 Fig. 6. — Diagrammatic restoration of the second thoracic limb (after Beecher). 

 EiG. 7. — Restoration of thoracic limb of Galymene senaria. 



en. Endopodite. ex. exopodite. ep. epipodite. (Bull. Mus. Comp. Zool. 

 vol. viii. 1881.) 

 Fig. 8. — Cephalic limb of Calymene x 3 ; supposed antennule. 

 Fig. 9. — Cephalic limb figured by Dr. Henry Woodward. (Quart. Journ. Geol. Soc. 



London, 1870, vol. xxvi. p. 487.) a. Side of hypostoma. 

 Fig. 10. — Slender jointed legs associated in same beds with Calymene at Cincinnati, 

 Ohio. 



1 The appendages of Triarthrus are replaced by iron pyrites, and are usually well 

 preserved. The specimens of Calymene and Ceraurus from the Trenton Limestone 

 of Trenton Falls, N.Y., were replaced by calcite, and in them there were preserved 

 even more delicate parts than I have yet observed in Triarthrus. Thin sections were 

 made of the latter and photographs obtained by transmitted light, that were used 

 in illustrating the paper in the Bulletin of the Museum of Comparative Zoology, 

 vol. viii. 1881. 



