94 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
interpretation is that it is difficult to imagine how the muscles could be so replaced that they 
happen to show in the section. Both the sections showing all four spots are evidently from 
the anterior part of the thorax, as they show traces of the abdominal sheath, which seems 
to be squeezed against the inside of the axial lobe, with the muscles (?) forced out to the 
sides. The ventral pair lie just inside the appendifers, but even if they are sections of muscles, 
all four are probably somewhat out of place. 
Hypostomial Muscles. 
The hypostoma fits tightly against the epistoma, or the doublure when the epistoma is 
absent, but in no trilobite has it ever been seen ankylosed to the dorsal test, and its rather 
frail connection therewith is evidenced by the relative rarity of specimens found with it 
in position. That the hypostoma was movable seems very probable, and that it was held in 
place by muscles, certain. The maculae were always believed to be muscle scars until Lind- 
stroem (1901, p. 8) announced the discovery by Liljevall of small granules on those of 
Goldius polvactin (Angelin). These were interpreted as lenses of eyes by Lindstroem, who 
tried to show that the maculae of all trilobites were functional or degenerate eyes. Most pa- 
laeontologists have not accepted this explanation, and since .the so called eyes cover only a 
part of the surface of the maculae, it is still possible to consider the latter as chiefly muscle- 
scars. 
In Lindstroem's summary (1901, pp. 71, 72) it is admitted that the globular lenses 
are found only in Bronteus (Goldius) (three Swedish species only) and Cheirurus splnu- 
losus Nieszkowski, while the prismatic structure supposed to represent degenerate eyes was 
found in eleven genera (Asaphidaa, Ilkenidae, Lichadidae). All of these are forms with well 
developed eyes, and Lindstroem himself points out that the appearance of actual lenses in 
the hypostoma was a late development, long after the necessity for them would appear to 
have passed. 
The use of the hypostoma has been discussed by Bernard (1892, p. 240) and extracts 
from his remarks are quoted : 
The earliest crustacean-annelids possessed large labra or prostomia projecting backward, still retained 
in the Apodidas and trilobites. This labrum almost necessitated a very deliberate manner of browsing.. The 
animal would creep along, and would have to run some way over its food before it could get it into its 
mouth, the whole process, it seems to us, necessitating a number of small movements backwards and forwards. 
Small living prey would very often escape, owing to the fact that the animal's mouth and jaws were not 
ready in position for them when first perceived. The labrum necessitates the animal passing forwards over 
its prey, then darting backward to follow it with its jaws. We here see how useful, the gnathobases of 
Apus must be in catching and holding prey which had been thus passed over. Indeed the whole arrangement 
of the limbs of Apus with the sensory endites forms an excellent trap to catch prey over which the labrum 
has passed. 
In alcoholic specimens of Apus the labrum is not in a horizontal plane, as it is in most, 
well preserved trilobites, but is tipped down at an angle of from 30 to 45 , and the big 
mandibles lie under it. It has considerable freedom of motion and is held in place by muscles 
which run forward and join the under side of the head near its posterior margin. It seems 
entirely possible that the hypostoma of the trilobite had as much mobility as the labrum of 
Apus, and that by opening downward it brought the mouth lower and nearer the food. It 
will be recalled that the hypostomata of practically all trilobites are pointed at the posterior 
margin, there being either a central point or a pair of prongs. By dropping down the hypos- 
toma until the point or prongs rested on or in the substratum, and sending food forward 
