32 BULLETIN OF THE 



fine longitudinal striae, these striae increasing in size and diminishing in num- 

 ber until we have such forms as Hyolithes hexagonus, Barrande, with only four 

 ridges in addition to the normal two lateral angles. In the specimens here 

 described, this number is reduced to two additional costee. The character 

 of the variation is quite distinct from that aiforded by a more acute or salient 

 ventral median line alone. 



Locality and position. — Station No. 2, North Attleborough, Mass., Cam- 

 brian, rare. 



12. Hyolithes communis, var. Emmonsi, Ford. 



Plate II. Fig. 17. 



Shell slender, straight, gradually tapering. In the specimen here described, 

 the part from the apical extremity to the imperforate septum is lost. It is fair 

 to presume that the length of this unseen part was about 15 mm.; the re- 

 mainder of the specimen is 38 mm. long, so that an occasional total length of 

 55 mm. or 60 mm. would not be too high an estimate. The apical angle is 11°. 

 The dorsal side is flattish, more or less depressed along the median line, the 

 depression becoming more distinct at a distance from the apical extremity. 

 The ventral side is quite evenly rounded, and is strongly convex; the lateral 

 angles are also rounded, their position chiefly defined by the depression along 

 the median line of the dorsal side. The cast of the septum warpedly curved; 

 the convexity turned towards the apex of the shell; otherwise smooth, with a 

 faint, raised margin. The surface of the shell is ornamented by fine transverse 

 striae. 



This species varies greatly in size, and the specimen here described is one 

 of the largest forms. The depressed median area of the dorsal side is most 

 characteristic. 



Locality and position. — Station No. 2, North Attleborough, Mass., Cam- 

 brian, one specimen. The type specimens were found near Troy, N. Y. 



13. Hyolithes Americanus, Billings. 



Plate II. Fig. 16. 



Shell straight, triangular, tapering gradually to an acute point. There is no 

 trace of a septum in the specimen at hand. The apical angle is 20°. The 

 dorsal side flattened or gently convex, with a slightly increased curvature at 

 the lateral angles. The lateral angles are but slightly rounded. The ventral 

 side is composed of two flat surfaces, which meet each other at the median line, 

 giving a triangular outline to the cross sections of the shell. The angle formed 

 at the median line is in type specimens never rounded; but there is a slight 

 tendency in some specimens to form a more or less distinct elevation or incip- 

 ient wing alone; this line. The flattened surfaces of the ventral side, meeting 

 sharply at the median lire, are characteristic of this species. The character of 



