71 



Colour. — The general colour is green, the dorsal area 

 ornamented with a number of longitudinal pale-green lines, 

 closely packed, and only discernible under lens. The rest of 

 shell decorated with a number of broad, dark-green longi- 

 tudinal dashes. The megalopores, in the form of pale-green 

 dots, are under a compound microscope visible on the anterior 

 and posterior valves and on the lateral areas, but not as marked 

 as in S. posidonialis . Inside of valves green. 



Anterior valve. — Slope very steep, without sculpture, 

 convex. In one specimen I counted 24 slits or grooves ; the 

 interior in this is irregularly fluted, each flute ending in a blunt 

 rounded tooth, a feature very distinct from any other known 

 member of this genus, or any other member of the Isehnochi- 

 tonidae, I believe. The slits are continued in shallow grooves, 

 indistinctly pitted. After photographing the interior of this 

 valve it met with a mishap, and is lost, and therefore I have 

 had to replace it with another from a specimen of considerably 

 larger size, which shows considerable differences ; the slits are 

 12 or 13, the teeth are blunt and irregular knobs, the fluted 

 appearance so much in evidence in the former valve is hardly 

 perceptible in this one ; perhaps with increased age the fluting 

 is filled in. 



Posterior valve. — Highly arched, mucro central, promi- 

 nent, posterior slope steep, nearly straight, but in type 

 becoming slightly convex as it nears the girdle, shell highly 

 polished, slits 13 to 15, a broad notch in the sutural laminae 

 of this valve. 



Median valve. — Uniformly smooth and glossy, the lateral 

 area strongly raised, and if looked at longitudinally with lens, 

 the anterior margin of this area is seen to be uneven, due to 

 shallow sulci following the growth lines ; so polished is the shell 

 that this undulating character of the margin of this area is 

 indiscernible unless the shell is held at a considerable angle. 



Median and, dorsal areas.— Smooth and dorsal area in 

 some valves slightly beaked, 1 slit. 



Girdle. — Clothed with small, closely-packed, smooth, 

 imbricating scales, circular in shape, and thick in proportion 

 to their diameter, reminding one of opalescent, flattened 

 pebbles. The dark line showing in plate follows the contour of 

 each valve. In life when looked at from above the only portion 

 of the girdle showing is the narrow strip between this line and 

 the shell. A darkish blotch extends across the girdle opposite 

 each suture and at irregular intervals round the anterior valve. 



Measurements. — Total length of type, 10 mm. (shown in 

 plate with girdle flattened out); breadth, 3'5 mm.; the - 

 specimen shown in plate with recurved girdle as in nature 



