FOSSIL INVERTEBRATES/Gastropods 381 Figure Some variations in shell form in the main groups of middle Palaeozoic gastropods (A) High spired shell: Murchisonia coronata (Murchisonioidea) (B) Trochiform, slit bearing shell: Devonorhineoderma orbignyana (Eotomarioidea) (C) Bilaterally symmet rical shell with a prominent selenizone: Kolihadiscus tureki (Cyrtolitoidea) (D) Turbiniform shell: Gyronema armata (Gyronematidae) (E) Openly coiled shell: Pragoserpulina tomasi (Pragoserpulinidae) (F) Discoidal shell: Stusakia pulchra (G) Sinistrally coiled shell: Vosko piella barborae (Onychochilidae) (H) Naticiform shell: Eifelcyrtus blodgetti (Vltavielidae) (I) Fusiform shell: Havlicekiela parvula (Perune loidea) High spired shells: (J) Pragozyga costata; (K) Palaeozyga bohemica (Loxonematoidea) (L, M) Bilaterally symmetrical shell: Bellerophon vasulites (Bellerophontoidea) (N) Limpet: Pragoscutula wareni (Pragoscutulidae) (P) Sinistrally coiled shell: Alaskiella medfraensis (Porcellioidea) (P) Discoidal shell: Nodeuomphalus labadyei (Euomphaloidea); (Q) Bilaterally symmetrical shell covered by secondary shell deposits: Branzovodiscus bajae (Bellerophontoidea) (A, B, D, F, O) Archaeogastropoda; (C) Cyrtonellida; (G) Mimos pirina; (H) Cyrtoneritimorpha; (L, M, Q) Bellerophontida; (I) Perunelomorpha; (E, J, K, N) Order uncertain; (P) Euomphalomorpha aragonitic shells with a crossed lamellar structure (Figures 6A and 6B) Some structural types are restricted to certain groups (e.g., nacre) and this may be used for their identification in fossils (Figure 6C) Nacreous and crossed lamellar structures have been known since the Palaeozoic Shell Coiling The majority of the shell-bearing gastropods have right-handed (dextral) shells, but some have left-handed (sinistral) shells (Figures and 2) Only a few gastropods have bilaterally symmetrical shells which may be uncoiled (limpets) or planispirally coiled (Figures 1M, 2C, 2L, and 2M) The limpetshaped shells were independently developed within all main gastropod groups from the asymmetrically coiled shells of their ancestors In contrast, planispirally coiled shells are known only in several groups, such as the Palaeozoic Porcellioidea and Bellerophontoidea (Figures 2L and 2M) or the Holocene Planorbioidea Some gastropods may change the coiling