MUSCULATURE. 
95 
to the mouth by means of the appendages, a trilobite could make of itself a most excellent 
trap, and if the animal could dart backward as well as forward, the hypostoma would be still 
more useful. There is no reason to suppose that they could not move backward, and the 
"pygidial antennae" of Neolenns indicate that animals of that genus at least did so. This 
habit of dropping down the hypostoma would also permit the use of those anterior gnatho- 
bases which seem too far ahead of the mouth in the trilobites with a long hypostoma. 
For actual evidence on this point, it is necessary to have recourse once more to Doctor 
Walcott's exceedingly valuable slices. From such sections of Ceraurus as his Nos. ioo, 106, 
108, 170, and 173, it is evident that the hypostoma of that form could be dropped con- 
siderably without disrupting the ventral membrane (fig. 30). Sections of Calymene already 
published (Walcott 1881, pi. 5, figs. 1, 2) show the hypostoma turned somewhat downward, 
and the slices themselves show sections of the anterior pair of gnathobases beneath the 
Fig. 30. — Longitudinal 
section of cephalon of 
Ceraurus pleurexanthe- 
mus, to show position of 
the mouth and folds of 
the ventral membrane 
between the glabella and 
the hypostoma. The test 
is in solid black and the 
part within the ventral 
membrane dotted. 
From a photographic 
enlargement. Specimen 
169. X 3-9- 
eo f >? tf ^ co 
I ^«Mllll,,„, -** 
■■= ""%. ***»% *' 
Fig. 31. — A copy of Doctor Moberg's figure of 
Nileus armadillo, showing the position of the 
muscle scars. 
hypostoma. When the hypostoma was horizontal, these gnathobases were crowded out at 
the sides. 
If the hypostoma were used in the manner indicated, the muscles must have been more 
efficient than those of the labrum of Apus, and it is probable that they crossed to the dorsal 
test. Just where they were attached is an unsolved problem. Barrande (1852, pi. 1, fig. 1) 
has indicated an anterior pair of scars and a single median one on the frontal lobe of 
Dalmanites that may be considered in this connection, and also three pairs of scars on the 
last two lobes of the glabella of Proetus (1852, pi. i, fig. 7). Moberg (1902, p. 295, pi. 3, 
figs. 2, 3, text fig. 1) has described in some detail the muscle-scars of a rather remarkable 
specimen of Nileus armadillo Dalman. While, as I shall point out, I do not agree wholly 
with Professor Moberg's interpretation, I give here a translation (made for Professor 
Beecher) of his description, with a copy of his text figure : 
The well preserved surface of the shell permits one to note not only the tubercle (t) but a number of 
symmetrically arranged glabellar impressions. And because of their resemblance to the muscular insertions 
of recent crustaceans, I must interpret them as such. They appear partly as rounded hollows (k and i), also 
as elongate straight or curved areas (a, b, c, e, g, h) made up of shallow impressions or furrows about 
1 mm. long, sub-parallel, and standing at an angle to the trend of the areas. Impression e is especially well 
