92 Walcott — Appcndnfjcs of the Trilobites. 



on the seven posterior thoracic limbs of Fig. 2 ; and in the spec- 

 imen from which Fig. 3 was drawn the limb opposite the tenth 

 segment from the pygidinm shows a slightly triangular second 

 (meropodite) and third (carpodite) joint. In Fig. 2, the limb a 

 is opposite the second free segment of the thorax anterior to the 

 pygidum. The limbs a and h, Fig. 3, clearly show that the four 

 proximal joints are broad and subtriangler in outline. A glance 

 at the abdominal swimming legs of the Phyllocarida (Parane- 

 balia), Schizopoda and Cumacea, suggests that the functions of 

 these legs were both natatory and ambulatory. 



The expododite illustrated by Beecher shows the dorsal sur- 

 face (Fig. 6). A number, presenting the ventral surface, are 

 shown on the right side of Fig. 2. They occur on the same 

 specimen as the endopodities, on the left side, but have been 

 pushed out of place. The most perfect is represented by m. 

 The jiroximal portion is formed of a rather large basal joint and 

 a number of short joints, 7 or 8. The distal end is formed of an 

 inner and outer segmented portion. The inner side is divided 

 into numerous segments by oblique divisions that give the im- 

 pression of a closely coiled spiral. The outer side is a cylindri- 

 cal, jointed, stem-like rim that is attached to the inner side, a 

 narrow, distinctly impressed line separating the two, except at 

 the somewhat flattened tij? where they merge into each other. 

 On the outer or upper surface of the outer side numerous crenu- 

 lations occur that extend into long setiB, 7i, Fig. 2; h, b, Fig. 1. 

 Dr. Beecher considers the expedite as a swimming organ ; but 

 from the manifest branchial character of the exopodite and at- 

 tached epipodite in Calymene (Fig, 7), it seems probable tltat 

 this exopodite of Triarthrus served largely as a gill, and that 

 the animal used the broad proximal joints of the posterior limbs 

 of the thorax as its principal propulsion in swimming. The 

 exopodite of Triarthrus looks like a consolidated exopodite and 

 ei^ipodite, very much as though these two organs as they occur 

 in Calymene were merged into one. 



Several specimens illustrate appendages beneath the pygidium. 

 Some have the broad proximal joints, d. Fig. 1, while others 

 show the outer rim of the exopodite c, Fig. 1. The material I 

 have seen indicates very little difference between the appendages 

 of the posterior half of the thorax and the pygidium, except 



