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 Ncolenus 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. 100, 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, i, 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- 



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 A pus, and it is probable that they crossed to the dorsal 

 test. Just where they were attached is an unsolved problem. Barrande (1852, pi. i, fig. i) 

 has indicated an anterior pair of scars and a single median one on the frontal lobe of 

 Dalmanitcs that may be considered in this connection, and also three pairs of scars on the 

 last two lobes of the glabella of Proctus (1852, pi. i, fig. 7). Moberg (1902, p. 295, pi. 3, 

 figs. 2, 3, text fig. i) 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 

 i mm. long, sub-parallel, and standing at an angle to the trend of the areas. Impression e is especially well 



