*Images in development
Key to Thracia and Cochlodesma
Crack on umbonal region, lithodesma protrudes ventrally. Small juveniles may not exhibit a crack, but lithodesma will always protrude ventrally = Cochlodesma.
No crack on umbo, lithodesma confluent with dorsal margin = Thracia.
If no crack but lithodesma is not confluent with dorsal margin and protrudes ventrally see above.
Lithodesma is oval-shaped, shell outline oval/subtruncate
Lithodesma peg-like, shell outline subtrigonal
From Arctic: Iceland, Northern Norway, Sweden
From British continental shelf
Elongate, rostrate and truncate posterior
Rounded triangular outline
Rostrum on posterior, visible as a shaded area on dorsal side
No rostrum on posterior and posterior end is longer than anterior
Prominent moon-shaped lithodesma
Short, thin lithodesma, often lost
Pallial sinus moderate - to beak
Posterior margin broadly curved
Posterior margin truncated/straight
Pallial sinus broad, points to anterior margin
Pallial sinus narrow, points to dorsal margin