Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 75. (Budapest 1983)

Vörös, A.: Some new genera of Brachiopoda from the Mediterranean Jurassic

Discussion : Bakonyithyris is ranged among the Zeilleriidae and Zeilleriacea because its loop is totally free from the median septum (which is, otherwise, extremely short). Some other loop characters (strong anterior spinosity, lood-like ascending branches) however bear certain resemblance to that of the terebratellaceans. Unfortunately, the earlier stages of loop development are unknown in Bakonyithyris therefore it cannot be stated whether the loop was connected to the median septum during early ontogeny or not, i.e. whether Bakonyithyris is a zeilleriacean or a terebratellacean. On the basis of thorough studies on loop development (BABANOVA 1964, BAKER 1972), it has been suggested that some typical zeilleriaceans (Aulacothyris and Zeilleria itself) should be removed from the Zeilleriacea to the Terebratellacea (RICHARDSON 1975). Perhaps this will happen to Bakonyithyris too, after a study of suitable series of juvenile specimens. At the present, however, we can use only the "principle" formulated ironically by DELANCE (1974, p. 375): "... les Zeilleriidae ... ne rassembleraient plus que les espèces dont l'ontogenèse du brachidium est inconnue, tandis que toutes les autres seraient rangées parmi les Dallinidae." Bakonyithyris has much similarity to Camerothyris and Aulacothyropsis but these are true terebratellaceans what is indicated, even externally, by their very long median septum. Among the Zeilleriidae (in traditional sense) Aulacothyris and Worohievella (= Woroboviella MUIR-WOOD, 1965, nom. null.) stand near to Bakonyithyris. Aulacothyris has elongate outline, carinate brachial valve and very long sulcus in contrast with the short subpentagonal outline, the evenly convex brachial valve and the short sulcus of Bakonyithyris. Internally, the development of the dental plates provides the most important difference: long, subparallel and closely spaced in Bakonyithyris, while short and divergent in Aulacothyris. Worobeivella is even more similar'to Bakonyithyris; externally they are hardly separable. Their internal features are also rather similar but Bakonyithyris has longer dental plates and a spinose loop; the crural processes are incurved in Bakonyithyris, while they are oppositely arched in Worobievella (cf. DAGYS 1963, fig. 99). Moreover, Worobievella is known only from the Upper Triassic. Distribution : Sinemurian and Pliensbachian of the Mediterranean Region (Sicily, Appennines. Southern Alps, Northern Limestone Alps, West Carpathians, Hungary, Crimea). Genus Securina gen. n. Derivatio nominis: axe-shaped; securis (Latin) = axe. Type species : S. securiformis (GEMMELLARO, 1874) Diagnosis : Medium sized terebratellids, trigonal in outline. Biconvex, with strong and long beak ridges in both valves meeting at anterolateral extremities. Planareas long, concave. Lateral commissures straight. Dental plates short; crural bases developing from inner socket ridges, crura falcifer(?>, loop long with wide ascending branches. Description : External characters (Representative figures of the type species : GEMMELLARO 1874, pl. X, fig. lOa-c, 11 ; BÖSE & SCHLOSSER 1900, pl. XVII, fig. 17a-b): Securina consists of a few zeilleriids with trigonal, '•axiniform" outline. The lateral margins are straight or gently curved outwards, the anterior margin is very expanded, straight or arched. The valves are equally convex in lateral view. Fold or sulcus are not developed. The planareas are well developed, large and long, generally concave, bordered by elongated beak ridges in both valves. The beak ridges generally meet at the lateral extremities of the anterior margin. The lateral commissures are straight, run in the middle (the deepest) line of the planareas. The anterior commissure is straight. The beak is strong, incurved; the foramen is mesothyridid. Internal characters (Fig. 19) — Pedicle valve: Beak interior is poorly known but the dental plates are short and seem to be divergent ventrally in cross section. The hinge-teeth are short and gently crenulated. Denticula are poorly developed. — Brachial valve (Fig. 21): A V-shaped septalium is present between the crural bases and is supported by a slender but short (less than one-third of the valve length) median septum. The septalium ends with the plane of articulation. The crural bases are given off dorsally from the inner socket ridges without insertion of outer hinge plates. The inner