OP SOUTHEEN INDIA. 391 



thickening, margin of shell in front below the beak insinuated; right yalve flat 

 or slightly convex near the umbo, with a small indistinct posterior and a still 

 shorter anterior ear, generally a little twisted and separated from the margin below 

 by a deep byssal sinus; hinge line in both valves straight, short, in the right 

 valve usually with a small, blunt tooth; ligament external, linear; muscular 

 scars small, posterior, sub-marginal, anterior placed near the ear and often 

 almost obsolete ; type. A, Fallasii, Keys. Most of the species of this genus are 

 recorded as occurring in Jurassic rocks, but Eichwald more recently considers the 

 beds from which most of these fossils had been obtained as Neocomien. Some species 

 are already known from the Lias, and others even from lower deposits appear to be 

 generically identical with Aucella. A few species also occur in cretaceous rocks, 

 but I am not acquainted with any tertiary species which could properly be referred 

 to the genus. Recent species, like A. smaragdina, ala-corvi, and others, for which 

 I would propose the sub-generic name JElectroma^ appear to represent Aucella 

 during the present epoch, but the form of the wing does not agree very well, and 

 the recent species are also less inequivalve. 



15. Avicula, Klein, 1753, CFteria, Scopoli, 1777). Shell obliquely sub-equi- 

 valve, the left val^^e being very often more convex than the right, pearly within, 

 with the outer surface more or less distinctly lamellated or striated ; hinge line 

 straight, and on both sides forming a more or less produced wing, the posterior one 

 being always longer than the anterior, hinge with a single cardinal tooth in each 

 valve, generally stronger in the left one, a byssal sinus is always present under 

 the short wing of the right valve ; ligament linear, external or partially sub-internal ; 

 posterior muscular scar enlarged, sub-central, anterior close to the ear and more or 

 less obsolete. 



15(^. The name Avicula has been restricted to the very oblique and poste- 

 riorly distinctly winged species, the shell has a rather thin structure, and the outer 

 surface is moderately lamellarly striated and ribbed ; the type is the Mediterranean 

 A, hirundo, Linn. 



155. The oblique, thin, and mostly smooth forms of the type of A, smarag- 

 dina, Reeve, could be separated under the sub-generic name JElectroma, They 

 closely resemble true AviculcB, but are more inequivalve, the right valve being 

 somewhat flatter, the hinge line is short, and the posterior wing very short, not sepa- 

 rated from the body of the shell. Mectroma is closely allied to Aucella, as already 

 stated. 



15(?. The third group form the true pearl-oysters of the type of Avicula mar- 

 garitifera, Linn., and for which Lamarck's name Meleagrina^ should be retained. 

 The shells when adult are generally of a solid structure and of a roundly squarish 

 form, the hinge line is long, with aflattened ligamental area externally ; the car- 

 dinal tooth is sometimes distinct, sometimes almost obsolete ; the posterior wing is 

 generally not distinctly separated from the body of the valves, but it is often 



* As the older name Margaritopliora or Margaritifera would involve a change in the specific appellation of the 

 type species, it seems preferable to retain that of M.eleagrina, which is also the best known. 



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