276 The America}!. Geologist. May, is98 
these primitive types and the detailed study of considerable 
material the following statements may be conceded to have 
the support of all facts at hand. 
Nothing is known as to the real nature or internal struct- 
ure of the earliest forms classed as gastropods, and in the 
absence of biologic evidence the only rational basis of classifi- 
cation is that of variation in form. 
Results based upon material of exceptional value for such 
investigation, and a tentative recast of the related forms de- 
scribed from the Cambrian, lead to the conclusion : 
1st, that the simple symmetrical cone was probalilv the 
earliest form of gastropod. 
2nd, that this form is represented by a group of fossils 
whose specific variation consisted in: 
a. Variation in highl. 
b. Variation of aperture in shape between the circle and symmetri- 
cal ellipse. 
c. Variation in striation, growth lines and radial striae being at 
most only specific characters and subject to obliteration in the process 
of fossilization. 
d. Variation in thickness of shell. 
Following the line suggested by a study of the material 
in hand there seems to be two steps in variation exhibited: 
1st, a tendency to acuminate aperture followed by or ac- 
companied by excentricity of apex. 
2nd, a tendency to a more irregular aperture usually 
more or less triangular or notched followed by or accompan- 
ied by more or less excentricity of apex. 
The first of these lines of variation gives rise to two di- 
vergent branches: ist, those anteriorly(acuminately) inclined; 
2nd, those posteriorly (obtusely) inclined or recurved. The 
first is the TrybliduDit type. The second is typified in a new 
genus Hypseloconus. 
Specific distinctions are chiefly questions of 
a. Size. 
b. Comparative excentricity of apex. 
c. Apical angle. 
d. Striations of all kinds. 
e. Variation from typical aperture. 
f. Comparative curvature of the sides. 
