78 



MOLLUSC A. 



[Nayades. 



2. Modiola GlBBSII, pi. XXVII, f. 7. 



Mudiola Gibbsii, First Ed., pi. 29, f. 7; Leach, MSS., p. 11 ; 

 lb., 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; lb., Sup., p. 65, pi. 26, f. 4; Brown, Ency. 

 Brit., p. 424 ; Mytilus discors, /l, 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. 



Mudiola 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; lb., 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 othor 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 Alga; and very often found lodged in the sub- 

 stance of the Ascidia 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 ; lb., 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 pcarlaceous; external surface covered 

 with a shining epidermis. 



