[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 43 



the middle, and one or more faint lonj^itudinal ridges along the two 

 halves of this side. Apical angle about 20°. 



Operculum sub-oval transversely, somewhat angulated on the ventral 

 side, and broadly rounded on the dorsal side, the nucleus is close to the 

 ventral side, and there are two strong furrows spreading Y-like on each 

 side of it and deepening toward the projecting lateral edges, the nucleus 

 and dorsal side are much elevated, and the ventral slopes behind the fur- 

 rows bent up. 



Sculpture. — Both sides of the tube s\n>w a granulated surface, inter- 

 rupted at intervals by beaded growth-lines. The operculum also has 

 lines of growth showing on its surface, especially toward the edges. 



Size. — Length to the apex, about 24 mm. Width, about 8 ram. 

 Operculum, 6x7 mm. 



This species may be compared with S. pennatulus, Holm, of the 

 Peltura Fauna of Sweden, which also has one strong rib and two weak 

 ones on the ventral side.^ It dilfers from H. americanus, Bill, in the form 

 of its operculum, and, therefore, in the shape of the tube. It is a larger 

 species than H. Billingsi. 



IIyolithes, sp. (PI. I., fig. 4.) 



This is a smaller species with very thin test, camerated toward the 

 apex. It changes its angle of aperture from 30° in the camerated part, 

 to 14° in the outer part of the tube. 



Sculpture. — On this smooth thin tube, no markings were detected. 



Size. — Length of the part known, 15 mm. Width of aperture, 5 mm. 



AGNOSTUS, Brongniart. 



AgNOSTUS MONTIS. (PI. I., fig. 0.) 



Agnostus, cf. integer. Barr., Roeminger, Proc. Acad. Sci., Philad'a, pt. i., 1887, p. 18, 



pi. 1, fig. 9. 



The species indicated by the above name occurs somewhat sparingly 

 in the Mount Stephen shales. It is evidently a Limbatus of the general 

 form of the earlier types of this group, but difiers from most of them in 

 the absence of lateral spines to the pygidium, and the rounded form of 

 this shield. Except for the more cylindrical rachis of the pygidium, this 

 species would be very near to A. Acadicus, Hartt. In some examples the 

 pygidium is wider in front, and the rachis also wider than represented in 

 the figure. 



I have no doubt that Roeminger described this species in his essay, 

 only he reversed the fossil and described the head for the tail ; correcting 

 the description in this respect it would read as follows : — 



"The glabella, or central convex protuberance of the head-shield, is 

 equilateral and not conical ; the anterior end of it is abruptly rounded 



1 Swedish Cambrian— Silurian Hyolithida», etc., p. 105, pi. 5, figs. 1 to 6. 



