316 MISS DONALD ON PROTEROZOIC GASTEROPODA [ May 1902, 
(an lit.), really agree with the former genus in the structure of the 
animal; or whether they possess distinctive and more primitive 
characters allying them with these ancient gasteropods. In stating 
this opinion with regard to the common origin of Murchisonia and 
Loxonema, Koken evidently refers more especially to M. insignis, 
Eichw. and allied species having a deep sinus in the outer lip, 
which I have shown’ to be distinct from the typical Murchisonia 
and members of the genus Hormotoma. It is, however, doubtful 
whether the true Mwrchisonie are derived from this stock. The 
type-species is of Devonian age, and varies greatly in its spiral angle, 
size, and ornamentation. Ihave examined the specimen of M. coro- 
nata, Goldf. (turbinata, Schloth.) in the collection of EK. de Verneuil, 
and it gives distinct evidence (as described by him) of a slit with 
parallel edges in the outer lip, the filling-up of which forms a band 
on all the whorls, bordered on each side by a keel. The peristome 
does not slope back so obliquely above the band, nor advance so pro- 
minently below, as in Hormotoma and Hetomarva, and there is a break 
in its continuity at the slit, so that there must have been three 
distinct areas of deposition of the shell, as in Plewrotomaria. The 
slit is not deep, being probably about two-fifths of the width of the 
body-whorl, and is therefore much shorter than that of the recent 
species of Plewrotomaria, Thus, in the structure of the outer lip and 
of the band, the Murchisonie come nearest to the Pleurotomarie ; but 
they are distinguished by being more elongated in form, and by 
having the aperture longer and slightly channelled below ; also they 
de not appear to have developed an inner pearly layer. The existing 
species of Plewrotomaria, contrary to the T'urritelle, are proved by 
recent investigations to be really primitive in structure. 
At present, I know of only one British Ordovician species of the 
Pleurotomariide which has the outer lip sufficiently well preserved 
to show the slit. It is in the collection of Mrs. Robert Gray, and 
is not only a new species, but is also probably referable to a new 
genus, for which I suggest the name Palwoschisma. The slit is 
short and narrow, being rather more than one-fifth of the cireum- 
ference of the last whorl in depth; it gives rise to a band bordered 
on each side by a keel, with a shghter submedian keel. Another 
shell has an opening in the outer lip preserved, but it has more the 
character of a deep sinus than a short slit; it would also be about 
one-fitth of the circumference of the body-whorl in depth if intact, 
although it is much wider in proportion than that of Palwoschisma. 
This shell most probably belongs to the genus Lophospira, Whitfield, 
and both it and the species of Pulwoschisma are of Llandeilo age. 
I shall not now consider the earliest British representatives of 
the Raphistomidw, as they do not possess a true sinual band, but 
shall proceed with the study of the more or less elongated forms 
having a band. Nor doI think it advisable at present to enter into 
the subject of the classification and the exact relationship of the 
' Quart. Journ, Geol. Soc. vol. lv (1899) p. 257. 
