64 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



is of course a question as to the number and the exact form of those on the pygidium, but 

 I think the present restoration is fairly well justified by the specimens. As would be ex- 

 pected from the narrow axial lobe, the gnathobases of the coxopodites are short and small. 



SUMMARY ON THE VENTRAL ANATOMY OF TRILOBITES. 

 COMPARISON OF APPENDAGES OF DIFFERENT GENERA. 



Since the appendages of Triarthrus, Cryptolithus, Neolenus, Calymene, and Ceraurus 

 are now known with some degree of completeness, those of Isotclus somewhat less fully, 

 and something at least of those of Ptychoparia, Kootenia, and Acidaspis, these forms being 

 representatives of all three orders and of seven different families of trilobites, it is of some 

 interest to compare the homologous organs of each. 



All in which the various appendages are preserved prove to have a pair of antennules, 

 four pairs of biramous limbs on the cephalon, as many pairs of biramous limbs as there 

 are segments in the thorax, and a variable number of pairs on the pygidium, with, in the 

 case of Neolenus alone, a pair of tactile organs at the posterior end. Each limb, whether 

 of cephalon, thorax, or pygidium, consists of a coxopodite, which is attached on its dorsal 

 side to the ventral integument and supported by an appendifer, an exopodite, and an endopo- 

 dite. The exopodite is setiferous, and the shaft is of variable form, consisting of one, two, 

 or numerous segments. The endopodite always has six segments, the distal one armed with 

 short movable spines. 



Coxopodite. 



The coxopodite does not correspond to the protopodite of higher Crustacea, the basip- 

 odite remaining as a separate entity. The inner end of the coxopodite is prolonged into 

 a flattened or cylindrical process, which on the cephalon is more or less modified to assist 

 in feeding, and so becomes a gnathobase or gnathite. The inner ends of the coxopodites of 

 the thorax and pygidium are also prolonged in a similar fashion, but are generally some- 

 what less modified. These organs also undoubtedly assisted in carrying food forward to the 

 mouth, but since they probably had other functions as well, I prefer to give them the more 

 non-committal name of cndobases. 



In Triarthrus and Neolenus the endobases are flattened and taper somewhat toward 

 the inward end. In Isotelus, Calymene and Ceraurus, they appear to have been cylindrical. 

 In other genera they are not yet well known. In all cases, particularly about the mouth, 

 they appear to have been directed somewhat backward from the point of attachment. As it 

 is supposed that these organs moved freely forward and backward, the position in which they 

 occur in the best preserved fossils should indicate something of their natural position when 

 muscles were relaxed. 



Cephalon. 



Antennules. Antennules are known in Triarthrus, Cryptolithus, Neolenus, and Ptycho- 

 paria. In all they are long, slender, and composed of numerous segments, which are spinif- 

 erous in Neolenus, and very probably so in the other genera. 



In Triarthrus, Neolenus, and Ptychoparia they project ahead of the cephalon, emerg- 

 ing quite close together under the front of the glabella, one on either side of the median 

 line. In Cryptolithus they turn backward beneath the body, but since only three or four 

 specimens are known which retain them, it is possible that other specimens would show 



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