Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . forming a concavity in the outline of the valve; it is calledthe lunula c. The hinge-ligament is sometimes between the umbones,never anterior to them. If the shell be divided by a line droppingfrom the umbo into an anterior (e/*) and posterior {f g^ part,it is never equally divided ; in other words, it is is least so : in Ghjcymeris and Solemya the anteriormoiety is longer than the posterior one; in almost all other bivalvesit is shorter, as in
Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . forming a concavity in the outline of the valve; it is calledthe lunula c. The hinge-ligament is sometimes between the umbones,never anterior to them. If the shell be divided by a line droppingfrom the umbo into an anterior (e/*) and posterior {f g^ part,it is never equally divided ; in other words, it is is least so : in Ghjcymeris and Solemya the anteriormoiety is longer than the posterior one; in almost all other bivalvesit is shorter, as in fig. 192., and commonly much shorter. MostLamellibranchs are equivalve, that is, the right and left valves are ofthe same size and shape. The exceptions occur in the stationaryand often fixed species, which lie on one side, when the lower valve isdeeper and more capacious than the upper one: this lower valve in theoyster. Pandora, and Lyonsia is the left valve, the smaller and flatterupper valve is the right one : in Chomostrea and Corbula, the left isthe smallest valve. Tiie Placume, Pectenes, Spondyli, and Aviculidce. LAMELLIBTIANCHIATA. 515 rest on the right valve : the A?iomicB are attached by degeneratedmuscular fibres passing through a hole or notch in that valve. Allthese shells are called iuequivalve. The bivalve is called close whenthe valves fit accurately ; it is gaping if part of the borders do notcome into contact when the shell is shut: in Gastroclicena this per-manent opening is anterior, and serves for the passage of the foot;in Mya it is posterior, and serves for the passage of the siphons ; inByssoarca it is at the ventral border, and serves for the passage ofthe byssus ; in Soleii and Glycymeris the shell gapes at both ends. The outer surface of the shell is usually coated by an uncalcifiedlayer of albumen called the periostracum or epiderm, continuous withthe mantle. This surface is variously ornamented in many species ;sometimes with ridges or ribs, which
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Keywords: ., bookauthorowenrichard18041892, bookcentury1800, bookdecade1850
