PULMONATA. 111 
a sinus somewhat resembling that in the shells of the Limacina, but greatly exag- 
gerated; this, most probably, is due to the accident which produced the distortion, or 
to some cause similar to that to which Mr. Gray attributes the sinus in Michaud’s 
A. sinuosus. The shell also is thicker than is usual in this genus ; but the shelly matter has 
been absorbed and replaced by carbonate of lime, and a slight thickening may have taken 
place in that process. It may be described as sub-conical, and much depressed, with the 
vertex about half way between the margin and the middle; the aperture is oblong and 
widely obovate. Inthe great depression of the shell this species resembles A. (Velletia) 
depressus, Desh.; but the aperture is more equally rounded at the extremities. 
Size-—Length about { of an inch; width, about 2-]0ths. 
Locality.— Sconce. 
Genus 15th. VELLETIA.* Gray. 
Acrotoxts, Beck, 1837. 
Vetietia, Gray, 1840. 
Gen. Char.—Dextral, with the apex turned sidewise towards the left margin; m 
all other respects resembling Ancylus. 
The dextral forms referred to Ancylus were first withdrawn by Beck, under the 
generic name Acro/oxus, but without any description; the genus was afterwards defined 
by Mr. Gray under that of Velletia. The animal, so far as its organisation is known, 
as well as the shell, resembles Ancylus, except that it is dextral and not sinistral ; 
and the genus has not been received without question, inasmuch as, apparently, it 
depended on a character insufficient in itself for generic distinction. Mr. W. Thompson, 
however, in his ‘ Remarks on the dentition of British Pulmonifera,’ to which I have 
before referred, states that, in their dentition, “dAncylus and Velletia present widely 
distinct characters, clearly showing that they do not belong to one genus. In Auxcylus 
there are thirty similar lateral teeth in a straight line on each side of the central tooth, 
and then there is a slight curve through a series of six more teeth, where a trifling 
change in their form occurs. In Ve//etia, on the contrary, no part of the horizontal 
row is straight; its central part is much arched, and is composed of the central tooth 
and twelve lateral teeth on each side, which do not alter much in form. Then comes 
one tooth of a different form, and lastly, six more on each side, which latter are in a 
slight curve.” <A closer examination of the comparative anatomy of the two animals 
will probably afford additional reasons for the separation of the present genus; in the 
meantime, I have retained it on the ground of the different characters of the dental 
apparatus recorded by Mr. Thompson. 
The living species are very few: one, /. dacustris, is found in this country; the 
* A name without signification, used by Mr. Gray on the principle advocated by Fabricius. 
