The Descent of Sonninia and Hammatoceras

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Abstract
Part I.— Sonninia . The genus Sonninia was founded by Bayle * to take the place of his Waagenia , a name which had been just previously used; and the type species was the same, Sonninia (non Waagenia ) propinquans † , Bayle. The species which are closely allied to Sonninia propinquans are Sonninia Sowerbyi (Miller), Sonn. Browni (Sowerby), Sonn. adicra (Waagen), Sonn. mesacantha (Waagen), Sonn. fissilobata , and, with the exception of the last, which was referred to Amaltheus ‡ , these species were located by Waagen in his genus Harpoceras . In this he was followed by Wright and many other authors ; but Neumayr § relegated them, or most of them, to Hyatt's genus Hammatoceras , and has been followed herein by Zittel ‖ and others. Haug makes two divisions of these species : — lst, Gruppe des Hamm . Sowerbyi ; 2nd, Gruppe des Hamm. Ogerieni ( Sonninia , Bayle); and his opinion is that the Inferior-0olite species, Amm. propinquans, Amm. patella , &c., are the direct descendants of the Liassic species, Amm. Ogerieni, Amm. navis , c. Thus far the history of the species embraced by the genus Sonninia . My present object is to show their probable descent, and by that means their correct relationship in a genealogical sense to various genera. The supposition that Sonninia propinquans was descended from Haugia Ogerieni takes no account of such a species as Sonninia acanthodes , n. sp. Whence has S. acanthodes derived its ventral furrows? and how, unless we suppose a case of atavism, has it acquired its circular (changing to quadrate)evolute whorls?

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