129 
timo angustato, basi confertius spiraliter sulcato, antice vix 
soluto; apertura verticali, ovali-rotundata ; perist. subdupli- 
cato, undique breviter expanso, superne subangulato. 
Long, 16, diam. 7 mill. 
Hab. 2 
4. On THE Genus MuLLERIA, SOWERBY, OR 
AcostT#A, D’ORBIGNY. 
By Dr. Joun Epwarp Gray, F.R.S., P.B.S., V.P.Z.S. 
(Mollusca, Pl. XXIX.) 
M. d’Orbigny has very kindly transferred to me the specimens 
of the shells which he described, in the ‘ Rev. et Mag. Zool.’ 1851, 
under the name of dcostea Guaduasana, and which he had received 
from Rio Sero near Guadual (Rio Magdalena) in Bogota. The ex- 
amination of the specimens proves the truth of the supposition which 
I formerly expressed, that Mr. Sowerby’s genus Miilleria was de- 
scribed from an imperfect specimen of this shell which had lost its 
umbones, with the young free state of the shell attached to them, 
in the manner so characteristic of this genus. 
The series of specimens consists of a pair, not in a very perfect con- 
dition, and without the produced umbo of the attached valve, like 
the specimen described by Mr. Sowerby, but in a less worn con- 
dition, four specimens of the attached valve, and several of the free 
upper one. 
The series of attached valves is curious, as showing the very dif- 
ferent state of the umbo, the manner in which the free valves 
are modified before one of the valves becomes fixed; also the man- 
ner in which the upper free valves separate from the free part by a 
natural crack, when the free valves become united together by their 
edges, forming a shelly tube. In two of the specimens this crack 
takes place almost immediately behind the posterior end of the sym- 
metrical free shell; in two of the others, the hinder part of the 
free shell is dilated into a triangular irregular portion before the 
hinder older part of the upper valve separates from the young one ; 
in one of these the triangular tube thus formed is narrow and elon- 
gate; in the other, broad, forming a nearly equilateral triangular 
cavity under the umbo of the attached valve. 
In three of the five specimens the shell is attached by the outer 
surface of the right valve; and in the other two by that of the left 
valve ; the three specimens attached by the right valve exhibit all 
the three variations in the form of the umbo, viz. the absence of the 
free shell (fig. 1.), the small (fig. 2.), and the large and much- 
dilated (fig. 3.) state of it. 
There can be no doubt, as far as one can undertake to determine 
from the examination of the shell alone, of the affinity of this genus 
with Htheria, from which it appears only to differ in the very small 
size, or indeed in the total. absence of the anterior adductor muscle. 
Like Etheria, the lower valve is rather attached to the stones and 
No. CCLXVITI.—Procrepines or tHE Zootoaicat Socimry. 
