Antennules.—Antennules are known in Triarthrus, Cryptolithus, Neolenus, and Ptychoparia. In all they are long, slender, and composed of numerous segments, which are spiniferous in Neolenus, and very probably so in the other genera.
In Triarthrus, Neolenus, and Ptychoparia they project ahead of the cephalon, emerging 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 that these organs were capable of being turned forward as well as backward. The proximal 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 curvature that is seen among the others only in Ptychoparia. The proximal end of an antennule 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 furrows. 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 Neolenus 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 endobase and an outward-directed pair of branches, the exopodite above, and the six-jointed endopodite beneath. The basipodite really bears the exopodite, but the latter also touches the coxopodite. This structure has been seen in Triarthrus, Cryptolithus, Neolenus, Kootenia, Calymene, Ceraurus, and Ptychoparia. In Triarthrus, Neolenus, Acidaspis, Ptyclioparia, 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 Neolenus 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 forward, and extend beyond the brim. The individual segments are flattened, probably compressed 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 appendages a paddle-like form similar to some of the appendages of eurypterids. The evidence 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, Neolenus, 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 function as mouth-parts (gnathites), and are especially modified for this purpose in Triarthrus, being flattened, shoe-shaped in outline, and so arranged that they work over one another in a shearing fashion. While the more anterior of the coxopodites are attached in front of the posterior tip of the hypostoma, the gnathites of Triarthrus bend backward so that all are behind the hypostoma. In Calymene and Ceraurus, two or three pairs of the gnathites are back of the hypostoma, and one or more pairs may be beside or under the hypostoma. In these genera the mouth is probably in front of the tip of the upper lip. In Isotelus, the mouth seems to have been situated in the notch between the two branches of the hypostoma, and the gnathites of two or three pairs of the appendages probably worked under its forks. Since the length of the hypostoma differs in the various species of Isotelus, there would be a variable number of gnathites projecting under its forks, according to the species. In this genus the gnathites are of the same long form, cylindrical in cross-section, as the endobases of the thoracic segments, but each is bowed back considerably from the point of attachment.
The gnathites of Neolenus are like the endobases of the thorax, but broader. The great length of the hypostoma makes it probable that the mouth was far back and that some of the gnathites were in front of it. The gnathites of Cryptolithus are unknown. Professor Beecher in his drawing shows some fragments with toothed ends near the hypostoma, and it may be that they are inner ends of gnathites, but I see nothing to substantiate such an interpretation. If, as some suppose, Cryptolithus was a mud feeder, the gnathites were probably poorly developed. Of the gnathites of Kootenia, Ptychoparia, and Acidaspis also nothing is known.
In each genus there is a pair of appendages for each segment of the thorax. When the axial lobe is narrow, the endobases of the coxopodites are small and short (Cryptolithus, Ceraurus, Calymene). When the axial lobe is wide, the endobases are long and stout (Isotelus, Triarthrus). The exopodites always lie above and in front of the corresponding endopodites. In Triarthrus the two branches are of practically equal length. In Cryptolithus the exopodites are much the longer. In Neolenus, Calymene, Ceraurus, Kootenia, and Ptychoparia, the exopodites are shorter than the endopodites.
The exopodites in Triarthrus consist of a proximal shaft, succeeded by numerous short segments, and ending distally in a long, grooved, somewhat spatula-shaped segment. Along the anterior margin of the shaft there are many small spines. Along the posterior margin there are numerous flattened setæ, which all lie in one plane and which seem to be more or less united to one another like the barbs of a feather. The setæ are short, not much longer than the width of one of the thoracic segments, and point backward and outward. In Cryptolithus the shaft does not seem to be made up of small segments, and is narrow, with a decided backward curve. The setæ are considerably longer and much more flattened than in Triarthrus. In Calymene the state of preservation does not allow a very full knowledge of the exopodites, but they appear to have a slender, unjointed shaft and short and delicate setæ. The coiled branches of the exopodites as described by Walcott seem to me to be only ordinary Triarthrus-like organs, and this, as I understand from Schuchert, was also the view of Beecher. In Ceraurus the exopodite seems to have been somewhat paddle-shaped, expanded at the distal end, and to have had rather thick, blade-like setæ.
The exopodite of Neolenus is decidedly leaf-like, and reminds one somewhat of the exites of some of the phyllopods. The shaft is a broad unsegmented blade. The setæ are slender, delicate, flattened, and a little longer than the width of the shaft. The exopodites of this genus point forward all along the body. In Kootenia the exopodites are like those of Neolenus, but with a narrower shaft. The exopodites of Ptychoparia appear to be very much like those of Triarthrus, but the shaft is probably not segmented.