Val. 58.) MURCHISONIA AND PLEUROTOMABRLA. 313 
18. On sox of the PRotERozotc GastEROPODA which have been referred 
to Murcarsonra and PLrEUROTOMARIA, with Duscriptions of 
New Suseenera and Species. By Miss Janz Donatp. (Com- 
municated by J. G. Goopcuixp, Esq., F.G.8. Read March 12th, 
1902.) 
[Puates VII-IX.] 
INTRODUCTION. 
Arrrr the creation of the genus Murchisonia by A. d’Archiac & 
K.de Verneuil in 1841, most of the Paleozoic, T’wurritella-like, banded 
gasteropoda were referred to that genus by later writers. Recently 
attention has been drawn to the fact that these shells do not all 
agree with the type, and that there are at least two separate groups, 
each distinguished by a different form of the outer lip. The typical 
group-is characterized by a slit of greater or less depth in the 
outer lip, with parallel edges, which is represented by a band on 
all the whorls. The other group is characterized by having merely 
a sinus in the outer lip, which in some cases gives rise to a band 
more or less distinctly limited, while in others it is not defined in 
any way. 
Some of the oldest-known gasteropoda, both elongated and short, 
have an opening in the outer lip; and as, owing to age and the 
manner of fossilization, very few have the outer lip well preserved, 
it is difficult to arrive at a correct conclusion as to its structure, 
more especially as the form of the lines of growth on the whorls 
rarely shows the actual depth of the sinus in the outer lip when 
mature. In the adult the outer lip frequently advances, thus 
rendering the mature sinus much deeper than the indications of it 
in the earlier stages of growth. The existence of a true slit in the 
outer lip, with a break in the continuity of the lines of growth, is 
still more difficult to ascertain where the outer lip is not visible; 
for the lines of growth often do not give evidence of the break, but 
sometimes even appear continuous, as if forming a shallow sinus, 
from the manner in which the shell is preserved. Again, there are 
cases, even where the outer lip is fairly well seen, in which it is 
difficult to decide whether the opening should be considered a slit 
or asinus. Where the lines of growth sweep back very obliquely, 
as in Hormotoma, Ectomaria, and allied genera, the sinus can be 
followed with tolerable ease. But where the lines of growth run 
less obliquely, asin the true Murchisonia and Lophospira, it is much 
more difficult to ascertain with accuracy (in the absence of the outer 
lip) whether there was a slit or not. 
With regard to these shells, two important questions require to 
be answered. Firstly: are forms possessing a slit, or those possessing 
a sinus, the more primitive ; and does the presence of a slit indicate 
a different line of development from that of the sinus, or is the one 
' Bull. Soc. Géol. France, yol. xii, p. 154. 
