NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP — KAUFFMAN 25 



spacial divisions of the valve (fig. 10c; see explanation). Discussions 

 are presented below for new terms employed, as a key to abbreviated 

 ratios (tables 1-5) or where it seems possible that misunderstandings 

 may arise in interpretation of a term in the descriptions as it is 

 applied to the Ostreidae. The letters arranged alphabetically below 

 refer to fig. 12, and tables 1-5. 



A: AATS — The ratio of the approximate area of the left (lower) valve to the 



approximate area of the attachment scar, as determined by their 



inscribing rectangles, oriented with their sides paralleling height 



and length. 

 A A — Anterior auricle, or ear (fig. 10a) ; rarely developed and generally poorly 



defined flattening and projection of the dorsoanterior margin, 



generally separated from the main body of the shell by a small 



auricular furrow or sulcus. 

 ANS — Anterior slope of the valve (includes lower part of the anterior auricle, 



where developed) ; the portion of the shell between the dorsal and 



anteroventral slopes, and the crest (fig. 10c). 

 A : P — The ratio of the area of a rectangle inscribing the valve to the number 



of plications present at the valve margin. 

 AS — Auricular sulcus (figs. lOa-c) ; an external furrow or depression which 



separates the anterior and posterior auricles from the main body 



of the shell. In this group it is commonly expressed as a rela- 

 tively deep, slightly enlarged interplical sulcus. 

 AVS — Anteroventral slope of the shell, between the median plication, crest, 



and anterior slope (fig. lOc). 

 CCP — Central cardinal plate. On the right valve, a flat surface developed in 



the central cardinal area, in place of a shallow resilifer. 

 CR — Crest of the valve ; the highest area of the valve, either slightly convex or 



nearly flat, situated dorsocentrally (fig. 10c). 

 D — Denticles ; small raised nodes, rounded to elongate, on the inner dorsolateral 



margins of the valves (fig. lOd). 

 DS — Dorsal slope of the valves (includes the attachment scar where present; 



fig. lOc). 

 HA — Hinge axis (cardinal axis), the axis of rotation of the valves (fig. lOd). 



The ligament lies dorsal, the main part of the resilium ventral. 



The only parts of the two shells always in contact. When the 



hinge line is straight, it may parallel this axis. 

 H : L — Ratio of valve height to valve length. 

 HP — High point of the valve (fig. 10c), marking the point of greatest convexity. 



This is usually within the crest on the median plication, and is a 



good reference point for delineating the various flanks of the 



valves. 

 H:DPAS — The ratio of the valve height to the greatest diameter of the 



posterior adductor muscle scar. 

 H : HATS — The ratio of the valve height to the height of the attachment scar, 



as measured on the left valve. 

 H : HD — The ratio of the valve height to the height of that part of the valve 



bearing denticles along the inner lateral margins. 



