VENTRAL STRUCTURE OF TRIARTHRUS 211 



(i, 2) are rather rudimentary in this genus, and are situated 

 further forward than in Triarthrus. The powerful mandibles 

 (3) are partly covered by the labrum, or hypostoma (hi/). 

 Then follow two well-developed gnathobases, representing 

 the maxilla3 (4, S), the more slender maxilliped (0), and the 

 large first thoracic limbs (7), behind which are the basal 

 endites, or gnathobases, of two of the phylloi3odous append- 

 ages (5, 9), The general similarity of the cephalic organs 

 of Apus and Triarthrus is quite apjDarent. The most con- 

 spicuous differences, as the absence of normal endopodial and 

 exoi^odial elements, disappear in a study of the ontogeny of 

 the limbs of Apus, thus bringing these organs in the two 

 groups into nearly exact correlation. 



There are, however, important structural features of other 

 parts of the body, which are quite dissimilar from Ajjus and 

 the higher Crustacea, and the exact relations of the trilobite 

 with any one group cannot be considered as fixed. Points of 

 likeness may be established with almost every order, showing 

 chiefly the relationship between the trilobite and the ancestors 

 of modern Crustacea. 



Summary of Ventral Organs of Triarthrus. 



A pair of appendages to each potential segment of the 

 animal, all of which are biramous except the anterior pair. 



Five pairs of appendages on the cephalon. 



Anterior antennse, or antennules, attached at the sides of 

 the hypostoma; simple, with a single many- jointed flagellum. 



First pair of biramous limbs, or posterior antenme, with 

 endopodite and exopodite; basal portion manducatory in 

 function. 



Second pair of biramous lindjs, or mandibles, similar to 

 preceding. 



Third and fourth pairs of biramous limbs, or maxilla?, same 

 as preceding, with large gnathobases, well-developed endopo- 

 dites, and fringed exopodites. 



Thoracic limbs biramous; endopodite a jointed crawling 

 leg; posteriorly the joints become flattened and leaf-like; 



