Systematic position and scope of Xenoconchia

Abstract

Unusual shells from the Carboniferous and Permian of Russia cannot be placed in currently accepted classes of mollusks. The author compares his material with that of recognized classes and concludes that it is so unique as to be placed in a new class Xenoconchia, within the phylum Mollusca. Members of the class are known from both older and younger rocks, but description is limited to two genera and four species, three of which are Permian. The shell of xenoconchian mollusks is tapering and closed at the apex. Steinkerns show septae are not present. Near the apex there is an interior annulation. Speculations are made about the soft parts of the animal. --E.L. Yochelson.

Permian Toxeumorphorida from Greenland: an appraisal of the molluscan class Xenoconchia

“…We concur that Toxeumorphoru and Pseudotoxeuma as described by Shimanskiy (1963) are not gastropods. There is clear homeomorphy, however, between these genera and extreme examples of P. (Orthonychia) which have lost all coiling and are essentially radially symmetrical.…”

“…Discussion by Starobogatov (1974) has led to confusion concerning the content of this correspondence. Shimanskiy (1963) Starobogatov (1974), after Yochelson had sent a cast to Shimanskiy, is illustrated (Fig. 7).…”

“…16: 4a, b). Starobogatov (1974) expanded the concept of the order Toxeumorphorida considerably from that of Shimanskiy (1963), in that four families in addition to the typical one were included. The family Pollicinidae as used contained only the type genus, the Middle Ordovician genus Pollicina Holzapfel 1895, which has remained a source of confusion since its original inadequate description.…”

“…Shimanskiy (1963) had placed Toxeumorphora and Pseudotoxeuma within the family Toxeumorphoridae, the only family in the order Toxeumorphorida, but Starobogatov (1974) restricted the family to the type genus and placed Pseudotoxeuma together with Dentoceras Ruzhencev & Shimanskiy 1954. Shimanskiy (1963), in proposing both Toxeumorphorida and Xenoconchia, discussed Dentoceras at some length but concluded that it should be excluded from the class Xenoconchia. According to Kummel (1964: K442) Dentoceras is a cephalopod with a slightly coiled shell in which the siphuncle is located close to the venter.…”

“…According to Kummel (1964: K442) Dentoceras is a cephalopod with a slightly coiled shell in which the siphuncle is located close to the venter. Septa, however, are absent in mature specimens of Dentoceras, although present in juveniles (Shimanskiy 1963), which may have prompted the assignment of Dentoceras to the Toxeumorphorida by Starobogatov (1974).…”