GASTROPODA. K>47 



A UlillillU-KlUlli ) 



trochonemoidrs and L. knoxrillfitsis on plate IAXI1 with almost any of 

 the species of Trochonema figured on plate I . \ \ V 1 1. As best suiting the purpose we 

 may recommend Trochmifmn Mlulutn, T. refrorsum 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 Lindstrom has placed in 

 the genus Trochus. For instance, we ask, what marked difference exists between 

 our Trochonema arctalum and T. obsolelum and Lindstn'un's Trochus dalli and '/'. 

 a t-iiyensis? 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 roust also. And what is there about all four of them that will justify 

 their separation from Trochonema? T. arctatum differs from our T. rolbinsi 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 oar 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. LindstrOm'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 Lindstrom save perhaps his T. fulminatus and 

 T. molds, the latter of which is a modified Cyclonema not far removed from ('. 

 transversum. As to whether these other species are true Trochid<e or not, we have no 

 opinion to offer at present, but some of them (e. g., T. lundgreni and T. stuxbergi) 

 doubtless stand in close genetic relationship with the Lower Silurian Raphittominu, 



TROCHONKMA UMBILICATCM Hull, and varieties. 



PLATE I.XXVII. KIOH. 1-4. 



PlmnXomarto ttmbiUeata HALL, 1847. I , \ V , vol. I, pp. 43 and 175. 

 TVoetoiMHM MnMfeofa SAI.TEK, 18M, Can. Oiy. Rern., Dec. I, p. 27. pi. vi. fig. 3. 



There are Mreral forms or varieties of this wi.l Suted speck-*. The one that occurs In th-- 



Tr.-nton 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-third* 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 subceotral ridge 

 or angularity enclosing the umbilicus is a nearly flat slope. The shell Is rath.-r thin and the apertural 

 margin In cast* Is much lea* eipanded than in the similar 2. btaeki. Lower and inner portions of p-'rt- 

 trvuie comparatively thin and not much reflected. Aperture very oblique. The surface marking* are 



