28o The American Geologist. May, i898 
Descriptio)!. Shell conical, aoerture oval, acuminate anteriorly; 
apex anteriorly excentric, submarginal or extra-marginal; surface 
smooth or striated, in the later representatives sometimes highly dec- 
orated; length of aperture usually exceeds hight of apex; shell variable 
in thickness; muscular attachment interrupted and the impressions ar- 
ranged in pairs. 
In many of the forms, obtained from the Cambrian rocks especially, 
no muscle attachment can be located. Those which show such impres- 
sions present a circle of paired scars around the cone. 
It would be an advantage to be able to limit the genus to a definite 
number of paired muscle scars. The question has been the subject of 
some disputes. In the earlier forms which are well enough preserved 
to show muscle scars the number is six. As it now stands, however, 
the form of the shell is the chief and almost the only feature ofTering a 
basis for a classification of many specimens. 
Tryblidium rectilaterale, n. sp. 
Plat? XX, Fiijs. 29 and \\\). Plate \XI. Fijf. 17. 
Shell conical; apex erect and anteriorly submarginal; sides straight 
forming a simple inclined cone; aperture broadly oval, anteriorly acum- 
inate and plane; surface shows strong growth plications; hight, Z2> miii.; 
length about 35 mm.; apical angle 50°x40°. The specimen is a cast of 
the interior. Shell thin, as indicated by space between cast and mould. 
No muscle scars preserved. 
Formation and locality: Dresbach. Found in the sandstone filling 
between boulders of the conglomerate at Taylor's Falls, Minn. 
Tryblidium convexum, n. sp. 
' . Plate XX, FiK.s. 24 and 2.i. Plate XXI, Fig. 18. 
Interior cast conical; apex erect and submarginal, acute, faUing just 
within the anterior margin of the aperture; aperture broadly oval, mark- 
edly acuminate anteriorly; dimensions proportioned approximately as 
follows: Length 40 mm.; hight 20 mm.; width 32 mm.; apical angle 
90°x6o°; sides a little convex, except the line from apex to anterior 
margin, which is almost straight; surface shows traces of growth lines 
and radial striations; a number of slight elevations and depressions lying 
in a band, midway between the base and apex of the cone, are taken to 
indicate muscle attachments. There are traces of six pairs of muscle 
scars in this form. 
This form is apparently very close to Metoptoma niobc Billings from 
the upper Calciferous, Missiquoi, Canada E. (Pal. Fos., I, 1862, p. 2)7)^ 
but is much larger and proportionately very much wider. 
Formation and locality: From the Dresbach at Taylor's Falls in the 
conglomerate. 
