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 macula: 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 polyactin (Angelin). These were interpreted as lenses of eyes by Lindstroem, who 

 tried to show that the macuhe 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 spinu- 

 losus Nieszkowski, while the prismatic structure supposed to represent degenerate eyes was 

 found in eleven genera (Asaphidas, Illaenidas, 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 Apodidae 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 

 wil| 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 



