GASTROPODA. 951 
Synopsis of Pleurotomariida.] 
remote affinities for each other.* Besides it leaves a large number of intermediate 
forms which may be placed with equal propriety into either genus. But this is not 
all, for we believe with other authors that, as now constituted, these genera embrace 
material more than sufficiently various to admit of defining a comparatively large 
number of valid generic groups,—valid in the sense that they are relatively as 
important groups of species as are ordinarily considered to be of generic value. 
Now, what are the characters upon which we propose to base these genera? 
First, upon the presence or absence and relative length ofa true slit, as distinguished 
from a mere apertural notch; second, the characters of the slit-band, among which 
its outline, as exhibited in transverse section, is the most important; third, the 
width of, and the position of the band and slit or notch with respect to the hight of 
the volutions; fourth, the form of the volutions, with respect to angularity and 
roundness; fifth, the form of the entire shell; sixth, the character and depth of the 
suture; seventh, the form and outline of the aperture as shown by the lines of 
growth; and eighth, the changes in the character of the volutions from the embry- 
onic to the mature stages. Corroborative and subordinate characters are furnished 
by surface markings, the apical angle, the relative size of the last volution, the 
presence or absence of an umbilicus, and minor peculiarities of the aperture, the 
notch, the slit and the band. The relative importance of these characters is not 
always the same, but they are sufficiently reliable for present requirements. 
SYNOPSIS OF GENERIC AND SUBORDINATE GROUPS OF PALEOZOIC PLEUROTOMARIID&.} 
I. Lopnosrira, Whitfield, 1886.{ Shells with more or less elevated spires; 
whorls closely coiled throughout or only in the upper part, the last often exhibiting 
a tendency to become disconnected; whorls angular on the periphery and bearing 
from one to five distinct carine; central or peripheral keel strongest and most 
prominent, carrying the band, which is obtusely rounded, or more or less distinctly 
trilineate, with the median line heavier and more prominent than the other two; 
axis rarely, if ever, solid; an umbilicus, usually of very small size, nearly always 
present. Inner lip generally thickened, often slightly twisted, turning around 
the umbilicus so as to form a kind of hollow pillar. Outer lip more or less deeply 
notched, but the center of the notch, which lies at the peripheral angle, is never 
prolonged into a slit. Surface markings parallel with the apertural edge; occasion- 
ally cancellated by fine spiral lines. Types, Murchinsonia bicincta Hall and M. 
serrulata Salter (—M. helicteres Whitfield, not Salter.) 
*To appreciate the subordinateness of the value of differences in the hights of the spire within moderate limits, a 
comparison of the figures of species of Lophospira described in this work should suffice. We pass by almost imperceptible 
gradations from species like LZ. ampla and L. notabilis, in which the apical angle is 80 and 90 degrees, to others, like 
L. bowdeni Safford sp.. in which the angle is of sometimes less than 25 degrees. And, so far as we can see, there is not the 
remotest chance for a generic separation between the first and the last of the species mentioned. 
+ Remarks on genera of which species are described in this work will be found on succeeding pages, 
+ Bull. Amer. Mus. Nat. Hist., vol.i, p. 312. 
