78 
MOLLUSCA. 
[Nayades. 
2. Modiola Gibbsii, pi. XXVII, f. 7. 
Modiola Gibbsii, First Ed., pi. 29, f. 7 ; Leach, MSS., p. 1 1 ; 
Ib., Miscell., II, p. 34, pi. 72, f. 2 ; Turton, Biv., p. 200, but 
not the figure; Fleming, p. 413. 
Shell somewhat triangular, compressed; beaks rounded; pos- 
terior side very short ; anterior side very long ; basal margin a 
little concave ; external surface regularly and transversely stri- 
ated, covered with a thick, umber-brown epidermis, which 
extends into foliations at the lines of growth ; these are pro- 
longed into filamentary processes, each of which is pectinated 
or notched down both sides, producing a bearded appearance ; 
inside of a yellowish-white. Length about two inches. 
This species is much more triangular than the young of M. 
papuana, and will easily be distinguished by the beard being 
simple in that species, while it is serrated in the present. Tur- 
ton has completely mistaken the figure of this species, and has 
given that of M. marmorata in its stead. 
Found on the Western coasts of England. 
3. Modiola discrepans, pi. XXVII, f. 8. 
Modiola discrepans, First Ed., pi. 29, f. 8 ; Lamarck, VI, p. 
114; Forbes, p. 44; Fleming, p. 413; Mytilus discrepans, 
Montagu, p. 169; Ib., Sup., p. 65, pi. 26, f. 4; Brown, Ency. 
Brit., p. 424; Mytilus discors, yS, Maton and Rackett, p. Ill, 
pi. 3, f. 9. 
Shell thin, suboval, compressed; external surface divided into 
three compartments, the posterior and anterior sides longitu- 
dinally ribbed, the middle area slightly wrinkled ; umbones 
obtuse and blunt, placed considerably towards the posterior 
side, which is much narrower than the broad and rounded 
anterior one ; whole surface covered with a thick, black epider- 
mis ; inside white, exhibiting iridescent reflections. Usual 
length at broadest part somewhat more than half an inch; 
breadth an inch and a quarter. It has been found two inches 
in breadth. 
Found on most of the British and Irish coasts. 
4. Modiola marmorata, pi. XXVII, f. 10. 
Modiola discors. First Ed., pi. 29, f. 10 ; Fleming, p. 413 ; 
Turton, Biv., p. 201, pi. 15, f. 4, 5 ; Modiola marmoratus, 
Forbes, p. 44; Mytilus discors. Pennant, IV, p. 240; Montagu, 
p. 167 ; Maton and Rackett, p. Ill, pi. 3, f. 8; Brown, Wer- 
nerian Mem., II, p. 515; Ib., Ency. Brit., p. 424. 
Shell very convex, somewhat cordiform; the posterior and 
anterior sides with about eighteen fine, longitudinal ribs, 
broader and stronger on the posterior side; the middle area 
plain, or slightly wrinkled transversely ; umbones large, promi- 
nent, obtuse at the point, turned to one side, and situate near 
the posterior side, which is rounded ; anterior side beaked, and 
slightly constricted; external surface covered with an olive- 
green, glossy epidermis, beneath which the shell is white, often 
mottled with pink or pale red ; inside white, tinged with pink ; 
margin provided with crenulations, which correspond with the 
exterior surface, which is ribbed, the other portions plain. Or- 
dinary length three-eighths of an inch; breadth five-eighths; 
sometimes, however, although rarely met with, seven-eighths in 
breadth. 
This species is common on all the British coasts, burrowing 
in the roots of Algcs, and very often found lodged in the sub- 
stance of the Asddia Mentula. 
This is not the M. discors of Lamarck, which is a foreign 
species, and very different from our shell ; and, therefore, I cor- 
dially approve of the name given to it by my friend Professor 
Forbes. 
Young shells of the M. discrepans have been confounded 
with this species, but the following distinctions will shew that 
they are widely different. The M. discors is very convex, 
nearly as thick as broad, and pointed at the anterior side ; the 
posterior side with about eighteen ribs ; while the M. discre- 
pans is depressed, broad and rounded at the anterior side, and 
with only eight or nine ribs on the posterior side ; the epider- 
mis is also more dull. 
5. Modiola Prideauxiana, pi. XXVII, f. 9. 
Modiola Prideauxiana, First Ed., pi. 29, f. 9 ; Leach, Zool. 
Miscell., II, p. 34 ; Ib., MSS., p. 1 1 . 
Shell subtransverse, a little compressed; striated from the 
umbo to the basal margin, with transverse, almost equidistant 
wrinkles; colour pale saffron-yellow externally, and white inter- 
nally, with a slight metallic lustre. Length about an eighth of 
an inch ; breadth a quarter. 
Found on the Devonshire coast, by Mr. Prideaux. 
6. Modiola rhombea, pi. XXXIX, f. 17. 
Modiola rhombea, Berkeley, Zool. Journ., Ill, p. 229, Sup., 
pi. 18, f. 1. 
Shell thin, subrhomboidal, gibbous, with transverse, subcre- 
nulated plications, and longitudinal ribs, which are somewhat 
obscure behind; umbones prominent, and incurved. Length 
from the umbo to the basal margin two lines. 
A single specimen of this interesting shell was dredged up at 
Weymouth, in August, 1826, adhering by its byssus to a large 
mass of slate. 
ORDER II.— DIMYAIRA. 
Shell provided with two separate lateral muscular impres- 
sions. 
Sub-Division I. — Lamellipedes. 
/ 
Family I Nayades. 
Shells inhabiting fresh waters ; the hinge sometimes with an 
irregular, simple, or complex tooth, and a longitudinal prolonged 
one, and sometimes none. Some have a compound muscular 
impression ; and the umbo is frequently decorticated. 
Genus 1 Anodon — Bruguiere. 
Shell equivalve, inequilateral, and transverse, for the most 
part very thin ; hinge line nearly straight ; hinge glabrous, with 
smooth lamina, destitute of cardinal teeth, truncated, or form- 
ing a sinus at the anterior end, terminating the apex of the 
shell; two lateral remote, muscular impressions, the posterior 
one being compound ; pallial impression entire, and seldom dis- 
tinctly marked ; ligament linear, external, sunk in a cleft at the 
anterior extremity; inside pearlaceous; external surface covered 
with a shining epidermis. 
