199 
Cyrtina sept0S((, T. Davidson, 1857, Mon. Brit. Carb. Bracdn, p. G8, pi. 14, fig. 1-10, and 
pi. 15, fig. 2. 
This sliell is transverse, and more or less rliomhoidal according to the 
age of the individual. The hinge line occupies the greatest breadth of the 
valves. The ventral valve is slightly curved, and has a shallow ventral 
furrow, well defined on each side by a fold rather thicker and more projecting 
than the six or seven folds which ornament the ventral furrow itself ; its 
hinge area is very large, triangular, slightly concave, with a deltoid fissure 
proportionately large ; its beak is nearly straight and not ^irominent. The 
dorsal valve is sub-semicircular, convex, with a slightly projecting dorsal 
ridge, and ornamentation corresponding to that of the ventral furrow.. The 
rest of the surface is covered with folds like those in the centre of the valves, 
varying in number with the individual ; these folds increase either by inter- 
position or by simple bifurcation, and are crossed by irregular lines of growth, 
often deep enough to produce imbricated plates. The inside of the ventral 
valve is furnished with two large, vertical, contiguous plates, which diverge 
as they recede from the beak towards the margin of the shell, and become 
re-united, and then again separate, one going to the right and the other to 
the left, thus forming the dental plates. The interior of the dorsal valve is 
unknown to me. 
Dimensions. — The only specimen sent me is five centimetres long by 
eight broad. The elevation of its axis is about 2‘5 centimetres. 
Relations and Differences. — In 1843 I confounded this species with 
S. subconicus, Martin, which it resembles only in the elevation of its area. 
I perfectly recognised this error later on, and have tried to repair it by 
furnishing my learned friend Mr. Davidson with the materials I had the 
opportunity of collecting. It is not impossible that Cyrtina dorsata, McCoy, 
is only a variety of C. septosa. 
Horizon and Localities. — In Belgium, this species occurs only in the 
upper beds of the Carboniferous Limestone, at Vise, but it is very rare, only 
a few examples having been found, some of which are in the Boyal Natural 
History Museum of Brussels. It is equally uncommon in England, where J. 
Phillips first discovered it at Bibblehead and Bartonhall, in Cumberland; 
since then it has been found at Park Hill, Longnor, in Derbyshire ; and at 
Settle, in Yorkshire. Mr. W. B. Clarke found a specimen of it in a light 
grey lime-stone on the Murrumbidgee, near Yass, probably having been 
brought a great distance by the waters of that river. 
