Thoracic Limbs. 91 



also suggested by a section of the limbs within the head of Caly- 

 mene, illustrated on Plate 1, Fig. 9, Bull. Mus, Comp. Zool., 

 Vol. 8, 1881. In this, a fifth limb is indicated close to the hy- 

 postoma. The trilobite was enrolled so as to include the anten- 

 nule entirely within the border of the head, A sketch, taken 

 from a photograph of the thin section by transmitted light, is 

 shown by Fig. 8, PI. 1. 



The hypostoma of Oeraurus* 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 Barande. 



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 

 extending 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 an 

 terior limbs are formed of a protopodite and a somewhat com 

 plex 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 endopodite of the anterior portion of the thoracic limbs 

 varies in the number of joints and in their relative length (Fig. 

 1, a, a). Two show four long proximal and three shorter dis 

 tal 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 pro 

 topodite. At e and f a new phase is indicated by the enlarge 

 ment 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 



*Loc. cit., PL iv., Fig. 5. 



