. Bulletin of the British Museum (Natural History), Geology. 118 BRUNTON AND MUNDY. Figs 73-77 Dorsirugatia tsagankhalgensis Lazarev & Suursuren. All from the type locality in Gobi Altai, Mongolia (see ). Figs 73a, b, the holotype viewed anteroventrally and posteroventrally showing the weak ribbing, which is lacking from the umbo. Palaeontological Institute, Moscow, 3385/1523. X3. Fig. 74, a dorsal valve exterior showing the late stage ribbing and lamellae. Palaeontological Institute, Moscow, 3385/1514. X3. Fig. 75, the external mould of a dorsal valve showing the weak r
. Bulletin of the British Museum (Natural History), Geology. 118 BRUNTON AND MUNDY. Figs 73-77 Dorsirugatia tsagankhalgensis Lazarev & Suursuren. All from the type locality in Gobi Altai, Mongolia (see ). Figs 73a, b, the holotype viewed anteroventrally and posteroventrally showing the weak ribbing, which is lacking from the umbo. Palaeontological Institute, Moscow, 3385/1523. X3. Fig. 74, a dorsal valve exterior showing the late stage ribbing and lamellae. Palaeontological Institute, Moscow, 3385/1514. X3. Fig. 75, the external mould of a dorsal valve showing the weak rugae and bases of lamellae. BD9423. X2. Figs 76a-d, anterolateral oblique view of a ventral valve exterior (X2), and the same in lateral, ventral and posterior views (X3). Arrows indicate spine bases. BD9421. Fig. 77, an anterolateral oblique view of an incomplete ventral valve and part of its internal mould showing tuberculation. BD9422. X2. additions. Ribs are lacking on the ears. Spines are sparse and confined to the ventral valve; their bases measure 0-2-0-3 mm in width. There are no spines at the hinge, but there is an indistinct row of up to three spines at the base of each flank; the first spine is at 3 mm from the beak, the second at 4—5 mm and the third at 7-7-5 mm. Dorsal valves are deeply concave and non-geniculate. The dorsal ornamentation is rugose and lamellose, with lamellae 0-2-0-4 mm wide posteriorly and wider anteriorly; thin growth lines may be visible. Costae are ill-defined and positioned anteriorly. Internally the ventral muscle markings are shallow and indistinct; teeth are absent and internal surfaces are tubercu- late. Ear baffles are low and broad, continuing anteriorly as weak marginal ridges. In dorsal valves the cardinal process is V-shaped, with no alveolus. The cardinal ridge is wide and indistinct. (The B factors above, in relation to valve inflation, repre- sent a logarithmic growth spiral in which low numbers indicate tighter spirals, gr
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