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Summary
In this paper the different interpretations of the genus Delthyris are discussed, and that given by Schuchert in 1913 is adopted with slight amendment. The species Delthyris elevata, D. crispa, D. elegans (nom. nov. for D. crispa, Hisinger, non Linnæus), and D. tenuilamellata, sp. n., from the British Silurian, are all described and figured.
The distinguishing characters of the genera Uncinulus and Uncinulina are pointed out, and Uncinulina is retained with U. fallaciosa as genotype. Rhynchonella stricklandi and Terebratula lewisi are transferred to the genus Uncinulina. Meristina tumida (Dalman) is shown to be a synonym of Meristina obtusa (Sowerby).
The British Silurian Spirifers can be separated into two groups:—(1) those with longitudinal folds and a transverse ornament of imbricating lamellæ (=Delthyris of Dalman) and (2) those with longitudinal folds bearing fine continuous longitudinal striæ (=Eospirifer of Schuchert).
The species referable to both these genera have as a rule been assigned to the genus Spirifer, from which, however, they differ in internal characters.