108 
5. On THE Genus MopIoLaRca. 
By Dr. Joun Epwarp Gray, F.R.S., P.B.S., V.P.Z.S. Erc. 
In the Synopsis of the British Museum for 1840, pp. 144, 155, 
I established a family of bivalve shells under the name of Crenellide, 
for the genera Crenella and Modiolarca, taking the character of the 
family from the animal of Modiola trapezina, the type of the genus 
Modiolarea, the only one that had then come under my examination. 
The following were the characters given :— 
«The family of Crenellide chiefly differs from the former (Myti- 
lide) in the mantle lobes being united together so as to leave only 
two posterior holes for the entrance and exit of the water, and a slit 
for the foot and beard. The hinge-margin is denticulated at each 
end, and the umbo is nearly central.” 
Shortly after, Mr. Alder described the animal of the British Cre- 
nella undulata, and found that the mantle was nearly as much open 
as that of the Modiole, and complained of the inaccuracy of the 
description ; the fact beg, that the two genera had very different 
animals, and that Crenella had been wrongly referred to the group, 
and taken as the patronymic of it. 
I have lately had an opportunity of verifying my description of 
the animal of Modiolarea, and find that it exactly agrees with the 
above character, and that it has a very peculiar-formed foot, most 
resembling that found in some drcide, which caused me to call the 
genus Modiolarca*. 
The following is a more particular description of the animal of the 
oe, which must be considered as the type of a peculiar family, 
calle 
Mopi1oLaRcaD®. 
Crenellide, Gray, 1840. 
Genus Mopiotarca, Gray, 1840. 
Mantle lobes united. Siphonal apertures two, distinct ; anal mo- 
derate ; branchial very large, inferior, simple edged ; pedal aperture 
small, basal, subanterior. Gills four, thick, dependent, subtrigonal, 
truncated in front, narrow, produced and united together behind. 
Lips four, moderate. Palpi obsolete. Foot oblong, base truncated, 
lower end lanceolate, acute in front, with a subposterior, central cavity 
for the byssus. 
Shell equivalve, oblong, thin; umbo subanterior. Hinge-teeth 
none, or rudimentary. Cartilage linear, external. Periostraca 
polished, hard. 
The animal lives attached to floating sea-weed. 
Modiolarca trapezinat=Modiola trapezina, Lamk. Hist. A. s. V. 
n. 17. ed. 2. vii. 24; Delessert, Icon. t. 13. fig. 7; Hanley, Cat. Re- 
cent Shells, 237. 
* Two genera have been made out of this word. Dr. Beck when in this 
country made a note that I had called the genus Modiolarca; but he appeared to 
have read it Modiolaria, and that name has been used for it. The latter name 
is now chiefly used for the more oblong Crenelle. 
+ Since the above was read, I find that M. Valenciennes has considered this shell 
as the type of a new genus, Phascolicama (see Gay’s Chili, 1854); and Mr. Gould 
has, more lately, formed a genus for it, under the name of Gaimarda.—J, E. Gray. 
