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 considerably without disrupting the ventral membrane (fig. 30). Sections of _Calymene_ already published (Walcott 1881, pl. 5, figs. 1, 2) show the hypostoma turned somewhat downward, and the slices themselves show sections of the anterior pair of gnathobases beneath the hypostoma. When the hypostoma was horizontal, these gnathobases were crowded out at the sides. [Illustration: Fig. 30.--Longitudinal section of cephalon of _Ceraurus pleurexanthemus_, 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. × 3.9.] [Illustration: Fig. 31.--A copy of Doctor Moberg's figure of _Nileus armadillo_, showing the position of the muscle scars.] 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, pl. 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 _Proëtus_ (1852, pl. 1, fig. 7). Moberg (1902, p. 295, pl. 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 marked, inasmuch as the perpendicular furrows are arranged in a shallow crescentic depression; and impression d shows besides the obscure furrows a number of irregularly rounded depressions. Larger similar ones occur at f, and in part extend over toward g. The meaning of these impressions, or their myologic significance, could be discussed, although such discussion might rather be termed guessing.