ID Keys

Thyasiroidea (500 - 5000m)

Shell subovate to subcircular (L>H) [c1, c2].
Go to #2
Shell outline oval (H=or >L) [c3], rhomboidal [c4] to polygonal (H>L) [c5].
Go to #9
Shell subcircular, posterior weakly angulate, all but smallest entirely coated with a rust coloured ferruginous deposit [c6].
Mendicula ferruginosa
Posterior distinctly more angulate than anterior, lacking deposits or only partially covered with ferruginous or black deposits.
Go to #3
Anterior and posterior dorsal margins sloping steeply [c7], posterior blunt [c8], gill with 2 demibranchs.
Go to #4
Anterior dorsal margin indistinct not sloping steeply [c9], posterior subacute [c10], gill with 1 demibranch.
Go to #5
Anterior dorsal margin straight [c11], posterior obliquely truncated [c12].
Thyasira tortuosa
Anterior dorsal margin curved [c13], posterior roundly truncated [c14].
Thyasira subovata
Dorsal margins either side of beaks minutely denticulate [c15].
Go to #6
Dorsal margins smooth.
Go to #7
Posterior profile roundly angulate [c16], no radial micro-sculpture.
Adontorhina similis
Posterior profile bi-angulate [c17], lower part of posterior margin somewhat truncated, radial micro-striations present [c18].
Adontorhina keegani
Prominent cardinal tubercle in left valve [c19, c20] (abyssal Norwegian Sea).
Mendicula ockelmanni
Taxon under review  
Hinge plate below beak slightly thickened [c21] but not with a prominent tubercle (upper bathyal).
Go to #8
Posterior subacute (Atlantic Margin) black deposit over dorsal areas [c22].
Mendicula sp 1
Taxon under review  
Posterior ventral margin somewhat truncated [c23] (Norwegian Margin), some entirely encrusted with black deposit.
Mendicula sp 2
[Taxon under review]  
Shell with dense radial rows of minute granules [c24].
Go to #10
Shell with commarginal sculpture or incremental lines only.
Go to #11
Outline polygonal [c25].
Parathyasira granulosa
Outline subcircular [c26].
Parathyasira subcircularis
Adductor scars thickened and raised, often showing white through the thinner shell [c27].
Genaxinus eumyarius
Adductor scars not thickened.
Go to #12
Outline quadrate ventral margin almost straight, posterior sulcus weak, auricle absent [c28].
Axinus grandis
Outline ovate to polygonal ventral margin rounded.
Go to #13
Submarginal sulcus present, auricle absent, [c29] posterior sulcus weak [c30].
Go to #14
Submarginal sulcus with auricle [c31], can be small or submarginal sulcus absent [c32].
Go to #16
Outline rhomboidal but scarcely higher than long, ventral margin broadly curved [c33], posterior area indistinct [c34].
Go to #15
Outline rhomboidal distinctly higher than long, ventral margin narrowly rounded [c35], posterior area distinct flat to shallowly excavated [c36].
Thyasira cf. rotunda
[Taxon under review]  
Subarctic samples [c37], larval shell smooth [c39].
Thyasira dunbari
Atlantic/Norwegian samples [c38], larval shells with radial striations [c40].

The distinction between these species requires further confirmation as well as comparison with high arctic examples of T. dunbari that are pyriform in outline.
Thyasira equalis
Ventral extremity in front of midline [c41], posterior outline bi-angulate [c42].
Go to #17
Ventral extremity in line with midline [c43], posterior bi-sinuate [c44] to evenly curved [c45].
Go to #19
Anterior hinge plate heavily thickened into elongate pseudo tooth and socket [c46], submarginal sulcus and lunule deeply excavated [c47]. Gill of a single demibranch.
Leptaxinus incrassatus
Hinge plate not greatly thickened, lunule and SMS not deep. Gill of 2 demibranchs.
Go to #18
Outline pyriform, prominent auricle making posterior outline distinctly biangulate, hinge with small tubercular tooth [c48], lunule flattened [c49].
Thyasira succisa
Outline ovate to pyriform, auricle low making posterior outline weakly biangulate, hinge lacking tooth [c50]. Lunule slightly elevated [c51].
Thyasira obsoleta
Posterior sulcus weak to absent.
Go to #20
Posterior sulcus prominent.
Go to #22
Anterior dorsal (lunule) margin very short, horizontal [c52]. Gill of a single demibranch.
Axinulus croulinensis
Anterior dorsal (lunule) margin long, sloping [c53].
Go to #21
Outline ventrally narrowed [c54] , sculpture of incremental lines only. Gill of a single demibranch.
Axinulus brevis
Outline broadly curved ventrally [c55], microscopically radially striated [c56]. Gill with both demibranchs.
Thyasira sp. 1
[Taxon under review]  
Anterior extended and narrowly rounded (beaked) [c57], anterior dorsal margin long sloping gently. Auricle high.
Thyasira sp. 2
Taxon under review  
Outline subcircular [c58], anterior dorsal margin steep, auricle long and low. (a large, cold seep species).
Thyasira scotiae