248 C. D. Walcott — Appendages of Trilohites. 



The hypostoma of Ceraunis ^ shows a rounded indentation of the 

 antero-lateral sides, where an antennule probably passed by it. 

 This character is strongly marked in Sao hirsuta, Proetus bohemicus, 

 Amphion fischeri, etc., as illustrated by Barrande. 



The character and position of the remaining cephalic limbs of 

 Triarthrus are not shown in any specimens that I have examined ; 

 but, from the relations of Calymene, Ceraurus and Triarthrus, 

 especially the two latter, it is probable that their arrangement is 

 essentially the same. 



Thoracic limbs. — Many specimens show the thoracic limbs extend- 

 ing out from beneath the carapace of Triarthrus. It was not until 

 by a fortunate dissection that I obtained the material illustrating 

 the limbs in position beneath the thorax. The anterior limbs are 

 formed of a protopodite and a somewhat complex exopodite. The 

 protopodite consists of a short basal and a long joint (Fig. 2, d, e), 

 to which the endopodite and exopodite are attached. This appears 

 to be direct in the posterior limbs of the thorax (Fig. 3, a), but as 

 yet the point of attachment of the basal joint of the exopodite has 

 not been seen in the anterior limbs. 



The endopodites of the anterior thoracic limbs vary in the 

 number of joints and in their relative length (Fig. 1, a, a). Two 

 show four long proximal and three shorter distal joints. Other 

 limbs show two smaller distal, and three or four proximal, while in 

 several there is a more or less uniform gradation from the protopodite 

 to the distal joint. In Fig. 1, some of these variations are indicated. 

 In Fig. 2, eleven limbs are shown, as seen from the under side. 

 The basal (coxal) joint is seen at b, d, e, and nine show the long 

 second joint of the protopodite. At e and / a new phase is indicated 

 by the enlargement of the proximal joints. This is marked in 

 a, b, c, d, and in Fig. 3 the details are more fully shown. These 

 joints occur on the seven posterior thoracic limbs of Fig. 2 ; and in 

 the specimen from which Fig. 3 was drawn the limb opposite the 

 the tenth segment from the pygidium shows a slightly triangular 

 second (meropodite) and third (carpopodite) joint. In Fig. 2, the 

 limb a is opposite the second free segment of the thorax anterior 

 to the pygidium. The limbs a and b, Fig. 3, clearly show that the 

 four proximal joints are broad and subtriangular in outline. A 

 glance at the abdominal swimming legs of the Phyllocarida (Parane- 

 baJia), Schizopoda and Cumacea, suggests that the functions of these 

 legs were both natatory and ambulatory. 



The expododite illustrated by Beecher shows the dorsal surface 

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

 the I'ight side of Fig. 2. They occur on the same specimen as the 

 endopodites, on the left side, but have been pushed out of place. 

 The most perfect is represented by m. The proximal portion is 

 formed of a rather large basal joint and a number of short joints, 

 seven or eight. The distal end is formed of an inner and outer 

 segmented portion. The inner side is divided into numerous seg- 

 ments by oblique divisions that give the impression of a closely 

 1 Zoc. cit. pi. iv. fig. 5. 



