C. E. Beecher — Ventral Integument of Trilobites. 173 



EXPLANATION OF PLATES. 

 Plate II. 



Triarthrus Becki Green. 



Figure 1. — A specimen viewed from the dorsal side ; showing the extent of 

 the antennules and the limbs on the right side. Enlarged about 

 three diameters. 



Figure 2. — The ventral side of a pygidium ; showing at the left of the 

 median line the form and disposition of the exOpodites and 

 endopodites. The conical ends of the joints of the endopodites 

 are provided with bundles of stiff hairs. Owing to the concavity 

 of the specimen, it is impossible to show it all in proper focus. 

 Enlarged ten diameters. 



Figure 3. — The posterior portion of an individual viewed from the ventral 

 side ; showing the distal ends of the exopodites, with their setas 

 and long fringes. Enlarged nearly ten diameters. 



Figure 4. — Dorsal view of an individual ; showing the nine pairs of anterior 

 thoracic limbs fully extended on the left side. The jointed 

 endopodites and fringed exopodites may be clearly differentiated. 

 Enlarged about three diameters. 



Figure 5. — A still further enlargement of some of the limbs of the preced- 

 ing specimen ; showing in more detail the distinctive characters 

 and arrangement of the exopodites and endopodites. Enlarged 

 about ten diameters. 



Utica slate, Ordovician, near Eome, New York. 

 This plate of illustrations, although very inadequately representing the 



actual objects, is introduced mainly to show the exquisite character of 



preservation of the specimens of Triarthrus. 



Plate III. 



Triarthrus Becki Green. 



Figure 1. — Ventral view of an individual ; showing the basal joints of the 

 antennules, the biramous appendages, and the series of gnatho- 

 bases. The appendages within the cephalon indicate their bira- 

 mous structure like those over the thorax. They are therefore 

 not simple as restored by Jaekel. The anal opening is shown 

 near the extremity of the pygidium, but is obscure on account 

 of not being in focus. Enlarged three and one-half diameters. 

 (Original of figure 1, Plate IV, vol. xv, American Geologist, 

 1895.) 



Utica slate, Ordovician, near Eome, New York. 



Plate IV. 



Triarthrus Becki Green. 



Figure I. — The ventral side of an individual prepared to show the character 

 of the endopodites of the entire thoracic series. The gnatho- 

 bases are distinctly seen extending obliquely inward from the 

 sides of the axis ; then follow, within the pleurosternal region, 

 the subtriangular joints of the endopodites with more slender 

 distal joints. The origin and course of the antennules at the 

 sides of the hypostoma are also shown. In the middle of the 

 axis of the mid-thoracic region, the ventral membrane is ex- 

 posed, and the transverse limitations of the sternal arches and 

 interarticular membrane may be observed. The arches show 

 the buttresses or ridges of apodemal nature, as described in the 

 text. Enlarged three and one-half diameters. 



Utica slate, Ordovician, near Rome, New York. 



