62 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



None of the specimens shows much more of the appendages of the cephalon than, the 

 hypostoma and the antennules, so that we are still in ignorance about the mouth parts. 



The most striking characteristics of the appendages are as follows : the antennules are 

 long, and turn backward instead of forward; none of the limbs projects beyond the margin 

 of the dorsal test; the exopodites extend beyond the endopodites, reaching very nearly to 

 .the margin of the test; the endopodites are not stretched out at right angles to the axis, but 

 the first three segments have a forward and outward direction as in Triarthrus, while the 

 last four turn back abruptly so that they are parallel to the axis; the limbs at the anterior 

 end of the thorax are much more powerful than the others; the dactylopodites of the endop- 

 odites show a fringe of setae instead of three spines as in Triarthrus and Neolenus. All 

 these would, as Beecher has already suggested, seem to be adaptations to a burrowing habit 

 of life, the antennules being turned backward and the other appendages kept within the 

 shelter of the dorsal test in order to protect them, and the anterior endopodites enlarged and 

 equipped with extra spines to make them more efficient digging and pushing organs. 



Restoration of Cryptolithus. 

 (Text fig. 20.) 



It should be definitely understood that the present figure is a restoration and not a 

 drawing of a specimen, and that there are many points in the morphology of Cryptolithus 

 about which no information is available, especially about the appendages under the central 

 portion of the cephalon. The information afforded by all the figures published in this 

 memoir is combined here. As gnathites are preserved on none of the specimens, those rep- 

 resented in the figure are purely conventional. 



A person who is acquainted only with Cryptolithus preserved in shale, or with figures, 

 usually has a very erroneous idea of the fringe It is not a flat border spread out around 

 the front of the head, but stands at an angle about 45 in uncrushed specimens of most 

 species. When viewed from the lower side, there is a single outer, concentric row of the 

 cup-shaped depressions, bounded within by a prominent girder. This row is in an approxi- 

 mately horizontal plane, while the remainder of the doublure of the fringe rises steeply into 

 the hollow of the cephalon. Since the front of the hypostoma is attached to this doublure, 

 it stands high up within the vault and under the glabella. Two specimens, Nos. 231 and 

 233, show something of the hypostoma, and they are the only ones known of any Ameri- 

 can trinucleid. That of specimen 233, the better preserved, is very small, straight across the 

 front, and oval behind. It seems that it is abnormally small in this specimen and I should 

 not be surprised if in other specimens it should be found to be larger. 



In the Bohemian Trinucleoidcs rcussi (Barrande), the oldest of the trinucleids, the hy- 

 postoma is very commonly present, and is of the proper size to just cover the cavity of the 

 glabella, seen from the lower side, and has, toward the anterior end, side flaps which reach 

 out under the prominent glabellar lobes. This large size of the hypostoma would cause the 

 antennules to be attached outside the dorsal furrows, and the position in which they are at- 

 tached in the American species of Cryptolithus may be explained as an inherited one, since 

 with the small hypostoma they might have been within the glabella, as in Triarthrus. 



The antennules are seen in three specimens, and in all cases are directed backward. The 

 particular course in which they are drawn in the restoration is purely arbitrary. The sec- 

 ond pair of cephalic appendages are represented as directed downward and forward, since 



