GASTROPODA. 1047 
Trochonema umbilicatum.] 
Lophospira trochonemoides and L. knoxvillensis on plate LXXII with almost any of 
the species of Trochonema figured on plate LXXVII. As best suiting the purpose we 
may recommend Trochonema bellulum, T. retrorsum and T. madisonense. It will be 
observed that the two sets of shells agree closely in all respects save one, viz.: the 
former has the continuity of the lines of growth interrupted by a true “band” on 
the upper peripheral carina, while in the latter the angle is simple and does not 
interrupt the continuity of the lines. 
Taking another set of species, Trochonema is brought into the closest kind of 
connection with some of the Upper Silurian shells which Lindstrém has placed in 
the genus Trochus. For instance, we ask, what marked difference exists between 
our T’rochonema arctatum and T. obsoletum and Lindstrém’s Trochus dalli and T. 
wisbyensis? There is none of any consequence that we can discover, so we are 
constrained to say if the first pair falls strictly within the limits of Trochonema then 
the second must also. And what is there about all four of them that will justify 
their separation from Tvochonema? T. arctatum differs from our TJ’. robbinsi and 
T. niota Hall sp., in having a narrower peripheral band. All of these three forms 
differ from the majority of the species of Trochonema in wanting the carina near the 
suture. But this deviation is surely not of more than specific importance since the 
absent carina is developed in our T’ simile, a shell that, with a general aspect like 
T. robbinsi, has the upper side of the whorls concave instead of regularly sloping 
and a carina beneath the suture. Lindstriém’s Trochus lamellosus also belongs to 
Trochonema. None of these species seems very intimately related to any of the 
other shells referred to Trochus by Lindstrém save perhaps his 7. fulminatus and 
T. mollis, the latter of which is a modified Cyclonema not far removed from C. 
transversum. As to whether these other species are true Trochide or not, we have no 
opinion to offer at present, but some of them (e. g., 7’. lundgreni and T. stuxbergi) 
doubtless stand in close genetic relationship with the Lower Silurian Raphistomina. 
TROCHONEMA UMBILICATUM Hall, and varieties. 
PLATE LXXVII, FIGS. 1—8. 
Pleurotomaria umbilicata HALL, 1847, Pal. N. Y., vol. i, pp. 43 and 175. 
Trochonema umbilicata SALTER, 1859, Can. Org. Rem., Dec. I, p. 27, pl. vi, fig. 3. 
There are several forms or varieties of this widely distributed species. The one that occurs in the 
Trenton limestone is usually regarded as the most typical of the species. It has rather low volutions, 
with a shoulder-like flat space bordering the suture and taking up about one-third of the width of the 
upper side, the remaining two-thirds being a concave slope to the peripheral band. This is almost exactly 
vertical and barely concave. The under surface from the lower peripheral angle to the subcentral ridge 
or angularity enclosing the umbilicus is a nearly flat slope. The shell is rather thin and the apertural 
margin in casts is much less expanded than in the similar 7. beachi. Lower and inner portions of peri- 
treme comparatively thin and not much reflected. Aperture very oblique. The surface markings are 
