196 DE. T. DAVIDSON ON EECENT BEACHIOPODA. 



upon themselves, and end in a small spiral directed downwards. Colour light yellow or 

 chestnut-brown. Length 15 lines, breadth 14^ lines. 



Hal). Great Ocean from Cobija (Bolivia) to the Island of San Lorenzo, Concepcion, 

 Chili (15 fathoms). It was obtained by Mr. F. H, Bradley at Callao, Peru, clustered 

 together in vast numbers and adhering in all stages of growth by its peduncle to the 

 surface of the shells of its neighbours, till a living mass of considerable breadth and 

 thickness was formed, living at a depth of six or more fathoms. 



Ohs. On the 17th of March, 1818, Mr. George B. Sowerby read before the Linnean 

 Society a paper on the genera Orljicula and Crcmia of Lamarck, in which for the first 

 time he introduced his Orbicula leevis, with good figures of the exterior and interior of 

 the valves. It can hardly be said that he described the species ; for all he said of it was, 

 " Orbicula leevis, 0. valvulis tenuibus Irevibus, tab. xxvi. fig. 1. Habitat in mari, saxis 

 adha^rens." In 1862, Mr. Lovel Reeve, in his monograph of Orbicula, states that " this 

 species was first described by Mr. Sowerby in a paper read before the Linnean Society 

 in December 1820, from a specimen attached to a grey flint or pebble nearly coated by 

 the roots of an Isis, of which the habitat was not known. Twelve years later it was 

 found attached to shells dredged by Mr. Cuming, as above noted, off Concepcion, Chili. 

 It is a stout horny shell, with surface smooth and faintly malleated. The vertex, which 

 much inclines posteriorly, is conically raised, swollen and rather obtuse." 



What we know of the animal of this species will be found recorded under Liscinisca 

 lamellosa. Having had before me a very large number of typical specimens and others 

 of Discinisca leevis, D. tenuis, and D. lamellosa, I am much inclined to consider them as 

 variations in shape, or varieties of a single species; but as malacologists generally seem to 

 differ with me in this respect, I will provisionally describe them separately. If a large 

 number of specimens of Discinisca leevis are examined, especially those from Callao and 

 Peru, it will be found that some of them are smooth and marked only by numerous fine 

 concentric lines. In some these lines are more strongly defined, and, again, in others they 

 are replaced by thin rows of adpressed or gently raised lamella3, which roughen the surface 

 of the shell. I am also of opinion that fig. 5 of plate i. of Beeve's Monogr. of Orbicula, 

 and referred by him to Orbicula tenuis, is no more than a smoother shell of Discinisca leevis. 

 If it should be therefore eventually found necessary to unite D. leevis, D. tenuis, and D. 

 kimelloset under a single denomination, the name leevis would have to be retained for the 

 species. 



108. Discinisca tenuis, Sowerby, sp. (Plate XXVI. figs. 12-17 a.) 



Orbicula tenuis, G. B. Sowerby, Tlies. Condi, vol. i. p. 3GG, pi. Ixxiii. figs. 4, 5, ]847; Davidson, Ann. 

 & Mag. Nat. Hist. 2ud ser. vol. ix. p. 376, 1852. 



Discinisca tenuis, Dall, Bull. Mus. Comp. Zool. Harvard, vol. iii. p. 41, 1871. 



Shell elongated, orbicular or circular, broadest anteriorly, thin, horny, semitransparent, 

 shining, smooth, marked only by fine concentric lines of growth. Colour light yellow 

 or pale yellowish brown. Upper or larger valve moderately convex, limpet-like; 

 apex small, slightly hooked, submargiual. Anterior half of lower or sinaller valve and 

 lateral portions of the posterior half convex, less so than in the opposite valve ; highest 

 elevation about the middle ; larger portion of the posterior half of the valve abruptly 

 sunken ; disk oval-shaped, commencing close to the posterior margin, extending to 



