Ltsianassa. 



CONCHIFERA. 



229 



Genus LXIIL— THRACIA— Z/McA. 



Shell very tliin, transverse, inequivalve, inequilateral, one 

 valvo usually more convex than the other ; beaks generally 

 obtuse, and sub-central ; hinge with a broad, transverse, fre- 

 quently thickened tooth in the centre, in which the cartilage 

 is situate ; surface covered with a very thin opiclermis ; two 

 well-marked but dissimilar muscular impressions in both 

 valves ; pallial impression interrupted by an arcuated sinus 

 at the posterior side, which is truncated. 



I. TuiiACiA OBLATA. — Tho Brought-up Thracia, pi. 

 XCIII. figs. 2, 3. 



Lutraria (?) oblata. Sowerby, VI. p. 66, pi. 534, 

 fig. 3. 



Transversely oval, compressed ; both sides obtuse and 

 slightly bent ; beaks prominent ; both siiles obtuse ; surface 

 small ; near tho posterior side a small keel ; pallial impres- 

 sion with a deep sinus. 



The London Clay^ Pegwell, Heme Bridge, and Bognor. 



2. Thracia depbessa. — The Depressed Thracia, pi. XCIII. 

 fig. 5. 



Mya depressa. Sowerby, V. p. 19, pi. -US. 



Obovate, compressed, very slightly gaping, anterior side 

 shorter than the other ; hinge-line straight and depressed ; 

 ligament external and short ; beaks prominent and incurved ; 

 surface smooth, with many undulating lines of growth ; sub- 

 stance of the shell thin. 



The Portland Stone, Brill, and Isle of Purbeck, and tho 

 Kimmeridge Clay, Weymouth and Specten. 



3. Thracia declives. — Bent-Down Thracia. 



F. declieis. Brown, Illust. Rec. Conch. Brit. p. 109, pL 

 4-t, fig. 5. 



Transversely oblong ovate ; rather compressed and thin ; 

 beaks large, very obtuse, and not quite central, that of the 

 latger valve with a hiatus for the reception of the beak of the 

 other valvo, not to prevent the opening of the shell ; posterior 

 side rounded ; anterior side truncated, with a shallow, oblique 

 furrow near the hinge line ; basal line nearly straight ; sur- 

 face strongly wrinkled, and irregularly striated concentri- 

 cally. 



The Red Crag, Sutton ; and tho Coral Crag, Ramshot. 



i. Thracia dubia. — The Dubious Thracia, pi. XCIII. 

 figs. 7, 8, 9. 



Transversely ovate ; beaka approximate ; a flexure from 

 the beaks to the base ; surface smooth. 



The London Clay, Bognor. 



Genps LXIV.— ANATINA.— Lrtmrt/T/t. 



Shell transverse, free, inequilateral, generally with unequal 

 valves ; sometimes gaping at both ends, and in some species 

 nearly close<l ; generally provided with a small accessory 

 spoon-shaped appendage, internally, in each valve, to which tho 

 ligament is attached ; connected with this, and also adhering 

 to the ligament, is a small irregularly-shaped testaceous inter- 



nal process, which serves to assist in strengthening the adhe- 

 sion between tho valves. 



1. Anati.va undulata. — The Waved Anatiua, pi. XC 

 fig. 30. 



Sanffiiinolaria undtilata. Sowerby, VI. p. 91, pi. 548, figs. 

 1, 2. Phillips, Geo. York, I. pi. 5. fig. 1. 



JIuch elongated transversely ; its width being considerably 

 more than twice its length ; thin, convex, roun<lod before ami 

 sub-truncatod behind; gaping slightly ; surface with transverse 

 undulations, which generally become obsolete towards the 

 posterior side ; slightly pearlaceous within. 



The Calcareous Grit, Malton and Brora : the Oxford Clay 

 and Cornbrash, Scarborough. 



Genus LXV.— LYSIANASSA.— 3/iinster. 



Shell thin, transverse, inequilateral, oval, convex, or ven- 

 tricose ; gaping at both sides ; surface ribbed ; those on the 

 cardinal margin anteriorly bent backwards, and the ribs on the 

 posterior side bent forw.ards, and radiated on the middle of the 

 back ; beaks sub-central ; hinge unknown. 



1. Lysianassa anguilipera. — The Angled Lysianassa, pi. 

 XCII. fig. 32. 



Mya anguilifera. Sowerby, III. p. 46, pi. 224, figs. 6, 7. 



Transversely elongated ; width nearly thrice its length ; 

 gibboso ; auterior side broadest and gaping ; posterior side 

 being small ; surface with obtuse angularly bent ridges, which 

 extend beyond the central portion, many of them reaching the 

 front without bending ; beaks a little produced, but obtuse. 



The Fuller's Earth, Smallcomb ; Bath and Bathford Hill. 



2. Lysianassa literata. — The Lettered Lysianassa, pi. 

 XCII. fig. 34. 



M. literata. Sowerby III. p. 45, pi. 224, fig. 1. 



Transversely elongated ; its width more than twice its 

 length ; sub-equilateral, convex ; surface smooth, with obtuse 

 angularly bent ridges, their angles upon tho central portion 

 in a longitudinal direction ; substance of the shell thin. 



Coral Rag, Malton ; Cornbrash, Scarborough ; the Inferior 

 Oolite, Coldmoor, Yorkshire. 



3. Lysianassa v. scripta. — The Letter V. Lysianassa, 

 pi. XCII. fig. 33. 



M. V. scripta. Sowerby, III. p. 46, pi. 224, f. 2, 3, 4, 5. 



Transversely elongated, sub-equilateral, convex, smooth, 

 with obtuse, angularly bent ridges upon the central portion ; 

 angles of the ridges acute, in an oblique direction. 



Distinguished from L. anguilifera, by the oblique direction 

 of the angles of the ridges, which are likewise more acute. 



There is another variety which has an oblique elevation bouDding the 

 anterior side. 



Tho Kclloways Rock, Wiltshire ; the Cornbrash, Bedford ; 

 and the Inferior Oolite, Claydon and Brora. 



4. Lysianassa rhombifera. — The Rounded Lysianassa, 

 pi. LXI.*** fig. 28. 



Ovatcly trapcziform ; narrow in front ; obliquely truncated 

 and gaping posteriorly ; beaks sub-central ; the ribs angulated, 

 truncated, and bent backwards. 



Tho Lias, Antrim, Ireland. 



