GASTROPODA. 955 
Synopsis of Pleurotomariida.] 
submedian on the last whorl; slit about one-fourth volution in length; aperture 
moderately oblique, the edge of the upper lip sweeping backward rather strongly, 
the lower broadly concave in the middle, the inner lip generally somewhat thickened 
and reflected. Surface with rather strong lines of growth; on one or both sides of 
the band often a more or less well-defined smooth space. Type, Bembexia larteti 
Munier-Chalmas sp. 
We have adopted this name for the group which in America is typified by the 
well-known Hamilton species, PJ. sulcomarginata Conrad, and includes also PJ. plani- 
dorsalis, Pl. adjutor, and Pl. nitella, described by Hall from the same formation, and 
Pl. shumardi Meek and Worthen, and Pl. elegantula Hall, two Lower Carboniferous 
shells, the first from the Keokuk, the second from the St. Louis. Excluding the 
spirally striated forms, whose relations to the group under consideration we cannot 
consider as established, Bembexia, as here defined, corresponds very nearly with 
Koken’s “Pleurotomarie interrupte.” The group, whether viewed as a genus or a 
subgenus is immaterial, seems to be a perfectly natural one. Provisionally we 
would give it an intermediate position between the Silurian Hotomaria and the 
Carboniferous EHuconospira, the tendency of variation exhibited by the species 
being toward the latter, while their general expression reminds one of the former. 
X. Movurtonra, De Koninck, 1883. Shell conical or somewhat discoidal, 
umbilicated. Band forming the periphery of the whorls, extremely prominent, 
thin, flange-like. On the upper side of whorls lines of growth curve backward 
without interruption from the suture line to the extreme outer edge; on the lower 
side first forward, then in a broad curve backward, and finally forward again as they 
turn into the umbilicus. Surface with revolving lines or not. Type, Pl. limata 
Lindstrém (Pl. carinata Sowerby). 
This type of shell is represented in American deposits by Murchisonia worthenana, 
a rather high species described by Miller from the Niagara limestone at Chicago. 
As now understood the principal peculiarity of Mourlonia, when compared with true 
Pleurotomariide, lies in the excessive development of the bounding plates of the 
slit-band. This particular feature reminds one of Huomphalopterus, Roemer, and it 
is possible that Mourlonia is really allied to that remarkable genus. Still, they are 
readily distinguished by the sigmoid instead of uniform curve of the lines of growth 
on the upper side of the whorls in Huomphalopterus. Despite the resemblances we 
are firmly convinced that the genesis of the two types is quite different. 
XI. Evconosprra, n. gen. (Ulrich.) Shell almost regularly conical, the base 
nearly flat, sometimes a little convex but oftener slightly concave; not perforated, 
though a small umbilical depression is always present; whorls rather numerous, the 
first three or four, so far as observed, less flattened on the upper or visible slope than 
