126 INTRODUCTION. 



Genus — Discina, Lamarck, 1819. 



Type — D. lamellosa, Brod., sp. Int., PI. IX, figs. 248 — 252. 



Orbicula, of Owen, Sow., &c., but not Obbicula, Cuvier or Lamarck-. 

 Discina, Lamarck, Gray, King, M'Coy, Philippi, &c. 



Shell circular, transversely or longitudinally oval ; upper or dorsal valve' conical, patel- 

 liform, with the apex inchning towards the posterior margin ; lower or ventral valve 

 opercular, flat, or partly convex, and perforated by a narrow oval longitudinal sht, reaching 

 to near the posterior margin and placed in the middle of an oval depressed disk. Valves 

 unarticulated, being kept in place by four thick adductor muscles, passing rather obliquely 

 from one to the other ; surface smooth, ornamented by numerous striae radiating from 

 the apex to the margin, or by concentric lines of growth, produced in the shape of 

 foliaceous expansions; shell structure horny, and perforated by minute tubuli. In the 

 interior of the perforated valve the muscular disk is more or less prominent, and from 

 the anterior extremity of the fissure a short longitudinal process arises. Four pairs of 

 muscular impressions are visible on the disk ; the posterior, submarginal pair, are probably 

 those of the sliding muscle ; the central four are the adductor scars, of which two lie on 

 either side of the foramen, while the anterior pair are situated more towards the centre of 

 the shell ; the fourth pair lie external to these. In the upper valve, the posterior adductor 

 scars are placed nearly horizontally, at a short distance from the margin, and are much 

 smaller than the anterior pair, which lie obliquely near the centre of the valve ; there are 

 no projecting processes in the interior of this valve. 



' In the letter addressed by Professor Owen to Professor M. Edwards, and published in the ' Ann. des 

 Sc. Nat.,' vol. iii, 3d ser., p. 319, 1845 j the learned Hunterian professor alludes to the valves of Terebratula 

 and Discina {orbicula of Owen), as follows : — 



"Pour en revenir au.x Tircbratules, j'ajouterai encore quelques mots relatifs a la mani^re dont j'envisage 

 les rapports de position des parties moUes et de la coquille. Dans le Terebratula flavescens, le pharynx est 

 eutoure d'un collier nerveux simple, et les principaux nerfs naissent de petits renflements situes aux angles 

 du cote de ce collier qui avoisine la base transversale des bras franges. Or, si le tube alimentaire etait 

 redresse' par le tiraillement de la bouche et du pharynx en avant, cette base transversale des bras, et les 

 points d'origine des nerfs analogues qui naissent ordinairement des ganglions sous-cesophagiens chez les 

 AloUusques, plus eleves en organisation, seraient situes du cote de la grande valve perforee. Je considere par 

 consequent cette valve comme etant la valve inferieure ou ventrale, et la position du coeur vient a I'appui 

 de cette opinion, puisque ce viscera se trouve plus pr^s de la petite valve ou valve dorsale que ne Test 

 I'intestine. Jadis, j'ai decrit I'intestin comme se terminant du cotd droit de la masse viscerale chez les 

 Terebratules aussi bien que chez les Orbicules (Discina). Je persiste encore dans cette mani^re de voir; 

 mais, dans I'Orbicule (Discina), la valve dorsale et imperforee est la plus grande et la plus convexe." 



