PARALLELODON ORNAT1SSIMUS. 155 



Parallelodon ornatissimus, de Koninck, 1885. Plate XIII, figs. 1 — 6. 



Paballelodon oenatissimus, de Koninck, 1885. Ann. Musee. Eoy. Hist. Nat. 



Belg., torn, xi, p. 161 ; pi. xxiv, figs. 1, 2 ; 

 pi. xxv, figs. 3, 30. 



Specific Characters. — Shell large, very inequilateral, tumid, transversely 

 oblong, wider in a dorso-ventral direction behind than in front. The anterior 

 end is comparatively short, comprising about one eighth of the valve ; it is 

 compressed. The anterior border makes a prominent right angle with the hinge- 

 line above, from which point it descends downwards and backwards, becoming 

 gradually semicircular ly curved below, where it joins the inferior border without 

 a break. The latter is almost straight and descends at first downwards and 

 backwards, but towards the posterior end gradually rises to meet the posterior 

 border at a very pronounced but blunted angle, which is produced backwards 

 considerably beyond the rest of the valve. The posterior border is straight, 

 obliquely truncate from above downwards and backwards, forming an obtuse 

 angle above with the hinge-line, which is straight and as long as the greatest 

 transverse diameter of the shell. 



The umbones are tumid, elongated, incurved, and pointed forwards, raised 

 above the hinge-line, widely separated and situated in the anterior fifth of the 

 valve. The ligamental area is comparatively very wide, and extends backwards 

 nearly the whole length of the hinge-line ; it is longitudinally striate in its whole 

 length. In front, this area is separated from the umbones in full-grown specimens 

 by a smooth surface ; and behind, the area is depressed below the extreme upper 

 border of the valve, which forms a raised, linear, longitudinal ridge external to it. 



The valves are much compressed in front, and the umbones rise suddenly, 

 being well marked off in front. From this point the valve becomes gradually 

 and convexly swollen, the greatest convexity being above at the level of the 

 ligamental area; posteriorly the valves are compressed and expanded in the 

 dorso-ventral diameter. There is an absence of oblique ridge, but the dorsal 

 slope is gradually rounded into the rest of the valve ; towards the posterior part 

 of the upper border the dorsal slope is slightly concave. 



Interior. — The anterior adductor muscle-scar is large, circular, shallow, and 

 bounded behind by a slight ridge. It is situated at the base of the umbonal 

 swelling, some distance within the margin of the valve. The posterior adductor 

 scar is large, circular, very shallow, situated on the dorsal slope some considerable 

 distance within the margin of the shell. The pallial line is entire and remote 

 from the margin. 



The hinge-plate is expanded in front, and bears a few small obliquely inclined 



