VENTRAL STRUCTURE OF TRIARTHRUS 207 



coxopodites, serving as gnathites (Plate VIII, figures 8-11). 

 They resemble each other, and are similar in form to the two 

 preceding limbs, though somewhat larger. They are usually 

 fairly well preserved, and their form and structure can be 

 approximately made out. The endopodites are composed of 

 stout joints, and could be extended but a short distance 

 beyond the margin of the head. The exopodites are more 

 slender, and carry an abundance of stiff seta3, which often 

 diverge in a fan-like manner from their line of attachment. 

 These brushes of setce occupj-ing the cavities of the cheeks 

 are often preserved in specimens where the other details of 

 the limbs are obscure or obliterated. In Triarthrus they are 

 evidently homologous w^ith similar brushes observed by Wal- 

 cott in Calymmene* 



This completes the number of paired appendages which can 

 be definitely referred to the head. It is evident they do not 

 differ conspicuously from each other, and, as will be presently 

 shown, they closely resemble the thoracic legs in all essential 

 structural characters. 



Thoracic Legs. — In the paper by the writer (J. c.) describing 

 the structure of the thoracic legs, the endopodites and exopo- 

 dites of the second and third pairs were illustrated, together 

 with their points of attachment. The form of the coxopodite, 

 or basal portion, was at that time unknown. With the 

 present material it is possible to add several details. The 

 most important are the inward prolongation of the coxopodite 

 of each limb toward the axial line, forming a gnathobase, and 

 the progressive development of this member. First it has 

 a slender cylindrical form in the posterior lialf of the series, 

 then becomes flattened and denticulate, and finally widens, 

 until on the head it forms the triangular plate-like coxopodite, 

 with masticatory ridge and functioning as a gnathite (Plate 

 VII, figure 1 ; Plate VIII, figures 1-4, 8-11). 



The large basal portions of the limbs of AsajihuH, in the 

 specimen illustrated by Walcott {Science, March, 1884), are 



* The Trilobite : Xew and ol<l Evidence relating to its Organization. Bull. 

 Mus. Comp. ZooL, VIII, No. 10, 1881. 



