COMPARISON OF APPENDAGES. 65 



that these organs were capable of being turned forward as well as backward. The proxi- 

 mal ends of the antennules being ball-like, it is probable, as Doctor Faxon has suggested to 

 me, that these "feelers" had considerable freedom of motion. The antennules of Triarthrus 

 are apparently somewhat less flexible than those of the other genera, and have a double cur- 

 vature that is seen among the others only in Ptyclioparia. The proximal end of an an- 

 tennule in Triarthrus is a short cylindrical shaft, apparently articulating in a sort of ball- 

 and-socket joint. The proximal end in the other genera is still unknown. The points of 

 attachment in Triarthrus seem to be under the inner part of the second pair of glabellar fur- 

 rows. In Cryptolithus they appear to be beside the anterior lobe of the glabella under what 

 have long been known as the antennal pits. In the other genera the location is not definitely 

 known, but in Neolcnus it seems to be under the dorsal furrows near the anterior end of the 

 glabella. Viewed from the under side, the point of attachment is probably always beside the 

 middle or anterior part of the hypostoma, just behind the side wings. 



Paired biramous appendages. Behind the antennules all the appendages except those 

 on the anal segment are biramous, consisting of a coxopodite with an inward-directed endo- 

 base and an outward-directed pair of branches, the exopodite above, and the six-jointed en- 

 dopodite beneath. The basipodite really bears the exopodite, but the latter also touches 

 the coxopodite. This structure has been seen in Triarthrus, Cryptolithus, Neolenus, Koo- 

 tenia, Calymenc, Ceraurus, and Ptychoparia. In Triarthrus, Neolenus, Acidaspis, Ptyclio- 

 paria, and Kootenia, the appendages extend beyond the margins of the dorsal test. In 

 Cryptolithus and Isotelus none (other than antennules) does so. In Isotelus and Acidaspis 

 only the endopodites have been seen. In Triarthrus, Calymene, Ceraurus, and Neolcnus 

 there are four pairs of appendages behind the antennules. The other genera probably had 

 the same number, but the full structure of the under part of their cephala is not known. In 

 Triarthrus the endopodites of the cephalon are slender, the individual segments parallel-sided, 

 the inner ones flattened, the outer ones cylindrical in section. They project slightly 

 beyond the edge of the cephalon when fully extended, and each terminates in three small 

 spines. In Cryptolithus the endopodites of the cephalon are longer than those of 

 the thorax, but with the possible exception of the first pair, are bent backward at 

 the carpopodite, and do not ordinarily project beyond the brim of the test. In Neolenus 

 the endopodites of the cephalon are rather thick and wide, but are long, project for- 

 ward, and extend beyond the brim. The individual segments are flattened, probably com- 

 pressed oval in section. The terminal segment of each is furnished with three strong spines 

 at its distal end. In Calymene and Ceraurus the endopodites appear to consist of slender 

 segments which are oval or circular in section. In Calymene Walcott believed the three 

 distal segments of the last endopodites of the head to be greatly enlarged, giving these ap- 

 pendages a paddle-like form similar to some of the appendages of eurypterids. The evi- 

 dence for this does not seem to me to be good. The cephalic endopodites of Isotelus are 

 entirely similar to those of the thorax, and are rather short, consisting of a series of short 

 cylindrical segments which do not taper greatly toward the distal end. The endopodites of 

 the cephalon of Acidaspis, Kootenia, and Ptychoparia are still unknown. 



The exopodites of the cephalon seem in all known cases {Triarthrus, Cryptolithus, Neo- 

 lenus, and Ceraurus) to be like those of the thorax. They point more directly forward in 

 most cases, project beyond the margin of the head normally only in Triarthrus, and usually 

 occupy the region under the cheeks (fixed and free). 



The endobases of the coxopodites of the appendages of the cephalon probably in all cases 



