C. E. Beecher — The Morphology of Triarthrus. 197 



The fringes on the exopodites in Triarthrus and Trinucleus are 

 made up of narrow, oblique, lamellar elements becoming filiform at 

 the ends. Thus, they presented a large surface to the external 

 medium, and partook of the nature of gills. But, as Gegenbaur 

 says, " the functions of respiration and of locomotion are often so 

 closely united that it is difficult to say whether certain forms of 

 these appendages should be regarded as gills, or feet, or both com- 

 bined." l For purposes of locomotion, the limbs of the cephalon and 

 pygidium were of feeble assistance compared with those on the 

 thorax ; and in the higher Crustacea, these two regions are the ones 

 where the greatest branchial specialization takes place. 



References. 



I. Beecher, C. E., 1893. — A Larval Form of Triarthrus. Araer. Journ. Science, iii, 



vol. xlvi, pp. 361, 362, November. 

 2. 1893. — On the Thoracic Legs of Triarthrus. Amer. Journ. 



Science, iii, vol. xlvi, pp. 467-470, December. Abstract of a 



paper " On the Structure and Development of Trilobites," read 



before the National Academy of Sciences, November 8th. 

 3. 1894. — On the Mode of Occurrence, and the Structure and 



Development, of Triarthrus Becki. American Geologist, vol. 



xiii, pp. 38-43, pi. iii, January. Abstract of a paper "On 



the Structure and Development of Trilobites," read before 



the National Academy of Sciences, November 8th, 1893. 

 4. 1894. — The Appendages of the Pygidium of Triarthrus. Amer. 



Journ. Science, iii, vol. xlvii, pp. 298-300, pi. vii, April. Read 



before the Conn Acad. Arts and Sci., March 21st. 

 5. 1895. — Further observations on the Ventral Structure of 



Triarthrus. American Geologist, vol. xv, pp. 91-100, 



pis. iv, v, February. 

 6. 1895. — The Larval Stages of Trilobites. American Geologist, 



vol. xvi, pp. 166-197, pis. viii-x, September. 

 7. Bernard, H. M., 1894.— The Systematic Position of the Trilobites. Quart. 



Journ. Geol. Soc. London, vol. l, pp. 411-432, August. Bead 



March 7th. 

 8. 1895. — Supplementary notes on the Systematic Position of 



the Trilobites. Quart. Journ. Geol. Soc. London, vol. li, pp. 



352-359, August. Read April 24th. 

 9. 1895.— The Zoological Position of the Trilobites. Science 



Progress, vol. iv, pp. 33-49, September. 

 10. Matthew, W. D., 1893. — On Antennae and" other Appendages of Triarthrus 



Beckii. Amer. Journ. Science, vol. xlvi, pp. 121-125, pi. i, 



August. Read before N. Y. Acad. Sci., May, and published 



in Trans. N. Y. Acad. Sci., vol. xii, pp. 237-241, pi. viii, 



July 22nd. 



II. Walcott, C. D., 1879.— Fossils of the Utica Slate. Trans. Albany Institute, 



vol. x, pp. 18-38, pis. i, ii, 1883. Author's extras printed in 

 advance, June, 1879. 



12. 1894.— Note on some Appendages of the Trilobites. Proc. 



Biol. Soc. Washington, vol. ix, pp. 89-97, pi. i, March 30th. 

 Read March 24th. Geological Magazine, n.s., Dec. IV, 

 Vol. I, pp. 246-251, PI. VIII, June. 



Explanation of Plate. 

 Fig. I.— Triarthrus Becki, Green: dorsal view; showing character and extent of 



antennules and limbs beyond the carapace, x 2\. 

 Fig. 2.—Triarth/us Becki, Green: ventral view; showing entire series of appen- 

 dages, together with hypostoma, metastoma, and anal opeuiug. x 2J. 

 Formation and locality.— Utica Slates, Ordovician: near Rome, New York, U.S.A. 



1 " Elements of Comparative Anatomy," Eng. edit. (Bell and Lankester), p. 241. 



