Notes on Carboniferous Lamellibranchs. 245 
The characters afforded by several well-preserved interiors 
of WV. attenuata showed, however, that there is no trace what- 
ever of any pallial sinus, and that in other respects, except 
the posterior attenuation of the shell, it does not differ from 
the recent Nucula. It is therefore proposed to return to the 
older name and to group JN. attenuata, N. sharmani, N. 
traquairt, and other allied forms with Mucula, of which, at 
the most, they form but a subgenus. WV. intermedia, and 
N. brevirostrum, Port., connect this subsection with the type 
forms. 
Dr Traquair has kindly lent several specimens of this 
genus for examination. 
It will be interesting to note, in tracing the history of these 
shells back in time, on what precise geological horizon the 
cartilage pit which distinguishes WVucula from Ctenodonta 
begins to appear; and, in continuing their history into 
Neozoic times, when the pallial sinus assumes a definite form. 
XVIII. Note on the genus Allorisma, King. By J. G. 
GoopcHILD, H.M. Geol. Survey, F.G.S., F.Z.S. 
(Read 20th January 1892.) 
[ ABSTRACT. | 
An examination of a large number of Carboniferous 
Lamellibranchs, variously referred to Myacites, Sanguinolites, 
and some other genera, confirms the view expressed by 
Professor King that these are referable mainly to the two 
genera Hdmondia and Allorisma. The author had examined 
a sufficient number of internal casts of various species of 
Allorisma to feel justified in proposing an extended descrip- 
tion of the genus, which is given as follows:—Genus 
Allorisma (Grammysiide). Shell equivalved, very inequi- 
lateral, umbones approximate, subterminal; extremities 
gaping ; hinge line nearly straight, occupying at least four- 
fifths of the entire length of the shell; ligament external, no 
internal cartilage; shell thin, strengthened by corrugations 
running parallel to the edges, and also by slight radial 
