Specific Characters in Orthoceras. — Foerste. 233 
considered as of later origin than smooth forms. Annular 
striations correspond to the striae of growth, the concentric 
ornamentation of brachiopods and lamellibranchs, and the 
transverse ornamentation of gasteropods. In their simplest 
form they consist of mere annular surface wrinkles, which 
within the limits of the same species may be distinct or obso- 
lete. They are quite commonly more distinct towards the 
larger end of the shell than towards the apical end. It is 
practically impossible to determine definitely at times 
whether there be two species at hand, one smooth and the 
other annularly wrinkled, or whether the specimens form only 
a single species. Safety lies only in the study of large col- 
lections of such variable forms. A more advanced type of 
this kind of ornamentation is represented b} T species in which 
the wrinkles have assumed the character of transverse or an- 
nular stria?, more or less equally distant, and at equal diam- 
eters of the shell, showing comparatively equal distinctness. 
But in the same way as the hight of the chambers will varj T , 
so the number of stria? in a given distance will be found at 
times to vary. 
Of quite a distinct type of transverse ornamentation 
though having their origin also in variations of growth, are 
the transverse annulations of certain types of Orthoceras. 
These are due not so much to the variation in thickness of 
the shell, as to the contraction and expansion of the diameter 
of the body cavity around which the shell is deposited. They 
correspond, after a fashion, to the stronger concentric corru- 
gations of brachiopods and lamellibranchs, but still better to 
the transverse or vertical ribs of gasteropods. These vary in 
their hight and prominence. In somewhat the same way 
longitudinal striae may be due to variations in the thickness 
of the shell along certain more or less equally distant vertical 
lines, as they maybe due to the form of the exterior portions 
of the body cavity within. Their relative degrees of ('(in- 
stancy has not been studied in connection with the preceding 
facts. 
The study of Orthoceras is still further complicated by the 
fact that in many instances the specimens of a locality exist 
only in the form of casts of the interiors of the chambers and 
siphuncle. While their discrimination is manifestly of con- 
