162 Professor C. E. Beecher — Structure of Triiohites-. 



These buttresses, or apodemes, include a single median one for 

 each mesosternite, with two others on each side extending forward 

 and obliquely inward, and enclosing sub-triangular or rhombic spa;ceSi 



The presence and disposition of these buttresses apparently afford 

 information regarding the ventral myology of the trilobites. A pair 

 of flexors is indicated, together with the laterah strands attached to 

 each mesosternite and extending forward and inward to their union- 

 with the main bundles within the cavity of the next anterior 

 somite. 



EXPLANATION OF PLATES. 



Plate IX. Triarthrus Becki, Green. 



Fig. 1.^ — A specimen viewed from the dorsal side; sliowing the extent of the 



antennules and the limbs on the right side. Enlarged ahouf threa 



diameters. 



Fis. 2i — 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. , . 



FlG. 3. — The posterior portion of an individual viewed from the ventral side; 



showing the distal ends of the exopodites, with their sette and ' long 



• _ fringes. Enlarged nearly ten diameters. - 



Fig. 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. ■ , , > , ; 



Fig. ' 5. — A still further enlargement of some of the limbs of the preceding. specimen ; 

 showing in more detail the distinctive characters and arrangement of the 

 exopodites and endopodites. Enlarged about ten diameters. , 



Utica Slate, Ordovician : near Rome, JSTew York. 

 This plate of illustrations, although very inadequately representing the actual 

 objects, is introduced mainly to show the exqiiisite character of preservation of the 

 specimens of Trwrif/w'ws. 



Plate X. Triarthrus Becki, Green. 



Ventral view of an individual; showing the basal joints of the antennules, the 

 biramous appendages, and the series of gnathobases. The appendages within 

 the cephalon indicate their biramous 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 fig. 1, pi. iv, vol. xv, 

 American Geologist, 1895.) 



Utica Slate, Ordovician : near Eome, New York. 



Plate XI. Triarthrus Becki, Green. 



The ventral side of an individual prepared to show the character of the endopodites 

 ^ ■ of ' the entire thoracic series. . The gnathobases are distinctly seen extending 

 obliquely inward from the sides of the axis ; then follow, within the pleuro- 

 sternal region, the sub -triangular 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 exposedj and the transverse limitations of the sternal 

 arches and interartictilar membrane may be, observed. The arches shoy?',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: 



