NAIADITES MAGNA. 151 



8. Naiadites magna, sp. nov., Bind. Plate XIX, figs. 17 — 20. 



Specific Characters. — Shell nioderately large, globosely subtriangular. The 

 anterior end is somewhat compressed and almost obsolete, and has the same 

 characters which obtain in other members of this genus. The anterior border is 

 convex, meeting the superior border at an angle which is almost a right angle ; 

 below it is produced downwards, joining insensibly the inferior border, which has 

 a downward and posterior direction, sinuated in front and pierced for the byssus. 

 Towards the posterior end the inferior border becomes deflected to the left, 

 forming a shallow depression with the concavity looking towards the right, 

 returning to the middle line at the posterior inferior angle, which is blunt and 

 often the situation of a triple S-shaped folding, the sulci of which are to be traced 

 upwards on the body of the posterior and lower part of the shell. The posterior 

 border is truncate from above downwards and backwards, making an obtuse angle 

 above with the superior border, and below is bluntly rounded in direction but folded. 

 The hinge-line is straight, and extends the Avhole length of the. superior border. 

 The umbones are obtuse and swollen, at first directed inwards to the middle line, but 

 before they reach it are bent sharply on themselves forwards, where they terminate 

 on either side of the anterior superior angle ; widely separated in casts, and 

 only slightly raised above the hinge-line. From the umbones a very gibbose and 

 broad oblique swelling, bounded in front by an almost obsolete oblique depression, 

 passes downwards and backwards, becoming broader as it nears the margin, near 

 which it becomes broken up into two or more ridges with intervening depressions, 

 the ridges in the left valve being anterior to those in the right and opposite the 

 depressions on the right valve. Above the oblique swelling the shell is rapidly 

 compressed and expanded upwards into the posterior part of the hinge-line, so 

 that the posterior slope of the shell is concave. The greatest convexity of the 

 shell is only slightly below the hinge-line. The inequality of the valves is not so 

 marked as in other species of this genus. 



The interior has a normal arrangement. Pallial line pitted, entire, and remote 

 from the edge of the shell. The anterior muscular impressions are trificl, the 

 most anterior being the largest. The position of the posterior adductor muscle 

 has not been exposed. There is a shallow groove on each side of the median line, 

 becoming deeper and broader posteriorly for the thickened lower margin of 

 the hinge. 



Exterior.— The surface is almost smooth, and covered with fine strias and lines 

 of growth ; near the margin the lines of growth are closer, rougher, and more 

 distinct. 



