W. D. Matthew — Antennce of Trilobites. 121 



and single, with important fluctuations, but not of epochal 

 significance, both during its advance and decline. 



The Cham plain subsidence of the land beneath its ice-sheet 

 probably affected the whole of the preglacially elevated area 

 before the growth of the ice-sheet was checked. For some 

 time the increase in the ice accumulation may have exceeded 

 the rate of depression, so that the surface of the thickening 

 ice-sheet continued to hold an undiminished altitude. But at 

 length the subsidence brought a warmer climate on the 

 southern border of the ice, causing it to retreat, and probably 

 giving to it a mainly steeper frontal gradient than during 

 its growth and culmination. To this steeper gradient and 

 consequently more vigorous glacial currents I attribute the 

 larger morainic accumulations of drift marking retreatal stages 

 than on the outermost drift boundary. When the ice had 

 considerably receded, the outer portion of the depressed area 

 was somewhat uplifted, to approximately its present height, 

 which it has since held, excepting minor oscillations. Gradu- 

 ally, as the ice withdrew from south to north, a principally 

 permanent wave of land elevation has followed, earliest uplift- 

 ing the loess region of the Mississippi basin, later the areas of 

 Lake Agassiz, of the Laurentian lakes, including Lake Cham- 

 plain, and of the St. Lawrence valley, and-latest the country 

 surrounding Iludson Bay, where this movement is still in 

 progress. The time since the departure of the ice there has 

 been too short, as in Scandinavia, to allow the earth's crust yet 

 to have completed its restoration to an isostatic condition. 



Art. XYII. — On Antennm and other Appendages of Tri- 

 or thr us Beckii ; by W. D. Matthew. 



[Read before the N. Y. Academy of Sciences. May, 1893.] 



Among the problems which paleontologists have in vain 

 tried to solve was, till a few years ago, that of the structure 

 and affinities of the trilobite. In all the vast numbers of these 

 animals which have been found and studied, scarcely any parts 

 have been preserved besides the dorsal shield and hypostome. 

 The legs, gills, and such other organs as they may have had, 

 have practically never been shown on any specimen. This is 

 chiefly because of the easy break afforded by the hard, smooth, 

 carapace, but partly also because of the character of these 

 organs, which seem to have been soft, easily disjointed, and 

 prone to maceration and decay. The only cases, so far as I 

 know, in which the organs of the under side have been defl- 



