248 THE GREAT BARRIER REEF. 



a remarkable resemblance to a fossil group of bivalve mollusca known as the Hippuritidae, 

 which are peculiarly characteristic of the cretaceous deposits. This resemblance is, however, 

 superficial, for it relates only to the corresponding disparity in the respective dimensions of 

 the right and left valves, and in the modification of the unattached valve in such manner that it 

 presents the form of a relatively small movable operculum. While all oysters (^genus Ostrea) belong 

 to that special group, the Monomyaria, whose shells are united by a single adductor muscle, the 

 Hippurites belonged undoubtedly to that distinct section, the Dimyaria, including mussels, cockles, 

 and the majority' of bivalves, in which two adductor muscles are invariably present. It is worthy 

 of note, however, in this connection, that, as recently shown by Dr. R. T. Jackson, in his 

 admirable thesis on the "Phylogeny of the Pelecypoda" (Memoirs Boston Soc. Nat. Hist., 1890), the 

 American oyster [Osfira virgiuiaua) at any rate passes through an intermediate embryonic condi- 

 tion when it possesses two distinct adductor muscles, while it has also a yet earlier developmental 

 phase, in which only one such muscle is represented. The significance of these developmental 

 facts is, that the oyster is in all probability the descendant of an ancestor corresponding 

 with the Hippurites in the possession of two adductor muscles, and that the Hippurites and all 

 allied Dimyarian types are in like manner the phylogenetic offspring of a primitive Monomyarian 

 ancestor. 



In the account of the Hippuritidae contained in Woodward's " Manual of the Mollusca," the 

 portion of an oyster closely resembling the type here introduced, is figured, under the title of 

 Ostrea cornucopia; a species originally established by Lamarck. The original figures of Lamarck's 

 type, however, given in the illustrated edition of the " Encjxlopedia Methodique," published in 

 the year 1827, represent a rostrate oyster, more nearly resembling a form, O. niordax, 

 illustrated by Fig. 2 of Chromo plate XIV., which is identified by the author with the Ostrea 

 cucullata of Born. In a footnote to the latest edition of Lamarck's " Animaux sans Vertebres," 

 Vol. VII., p. 230, 1836, the editors maintain that Ostrea cornucopia: and O. cucullata, as 

 represented by the Paris Museum types, are varieties only of one and the same species. In the 

 same manner that the beaked 0. cucullata has been shown to represent a variety only of 

 the ordinary Ostrea uiorda.x, the very elongated tubular form here figured and described, is 

 regarded by the author as representing an exaggerated growth of the variety cucullata. 

 From an examination of a very considerable number of specimens of all ages, derived 

 from different localities, it has been found possible to arrange an unbroken series from the 

 typical Ostrea iiionlax through the beaked varietj', culcullata, to the elongate tubular form. Under 

 these circiunstances it is desirable to associate it only with a title that shall indicate its 

 position as a remarkable variety of Ostrea iiiorda.x, and in that association I here con- 

 fer upon it the distinctive name of 0. morciax, variety cornucopiaformis. Many other oysters, 

 it maj' be remarked, exhibit the same tendency of the attached valve to become abnormally 

 elongated and camei-atcd. Examples in the teaching collection of the Koyal College of 



