Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 16. 1993. (Budapest, 1993)

of its basically uniplicate character, the anterior commissure can rather be termed parasulcate. This anterior commissure corresponds to a dorsal fold and a gentle ventral sulcus The shells are almost completely smooth, except the growth lines and a few, irregular, longitudinal wrinkles on the planareas. Two longitudinal ridges on the ventral valve and corresponding furrows on the dorsal valve run from the umbo to the lateral deflections of the anterior commissure. Internal characters (Text-fig. 1) - Pedicle valve: The delthyrial cavity is trapezoidal in cross-section, showing a narrow ventral groove; the umbonal cavities are relatively small and drop-shaped in cross section. The thin dental plates are divergent ventrally and are reinforced with callous secondary shell material in the incurved part of the umbo; closer to the plane of dentition they become subparallel and loose contact with the floor of the valve. The well-developed pedicle collar is about 5 mm long and 1 mm across, and follows the curvature of the beak, suggesting the presence of an active pedicle, at least in the early adult stage (the sectioned specimen was 34 mm in length). The whole umbonal part is characterized by strong, irregular secondary thickening with callus. The deltidial plates are fused, forming a symphytium. The hinge teeth are massive, expand dorsally, partly crenulated. Denticula are present. - Brachial valve: The incurved beak continues in a slightly crenulated blade-like structure which may be termed as cardinal process. The fused inner hinge plates form a septalium which can be seen in a gently overturned position in the cross sections, due to the curvature of the beak. The septalium is supported by a well-developed, long median septum. The outer hinge plates are massive, steeply dipping outwards. The inner surface of the hinge sockets is crenulated. The crural bases emerge dorsally and give rise to crura of radulifer type persisting to the half-length of the valve. Adductor muscle impressions have not been observed. Remarks: This species stands alone within the genus Lessiniella and there are no other Jurassic brachiopod species to be compared with it. Distribution: So far Lessiniella benettii is known only from the Callovian of the Lessini Mts. (Southern Alps, Italy). NOTES ON THE PALEOBIOGEOGRAPHY AND PHYLOGENY OF THE LATE MIDDLE JURASSIC GIANT RHYNCHONELLIDS Jurassic rhynchonellids are generally not very large, therefore the almost synchronous appearance of the giant Septirhynchia and Lessiniella in the Callovian is somewhat surprising. A third group of very large rhynchonellids, the group of "Rhynchonella" decorata (Schlotheim) appeared a little earlier (Late Bathonian). This species was placed into the officially invalid genus "Isjuminella" by Drot & Fischer (1966) and later to Burmirhynchia by Dardeau & Laurin (1982). A better attempt was made by Taddei Ruggiero & Ungaro (1983) who erected a new genus (Sardorhynchia) for similar, giant rhynchonellids from the Bathonian-Callovian of Sardinia and attributed, though not explicitly, "Rhynchonella" decorata to Sardorhynchia. Here we may speak about a late Middle Jurassic episode of gigantism among the rhynchonellids. May the synchroneity involve a common cause ? Gigantism can be due to different environmental factors (optimum circumstances, especially food-supply, isolation, deep-sea environment) and to special phylogenetic relationships, therefore it seems to be almost hopeless to find the