24 
SILURIAN MOLLUSCA. 
eccentric, and placed near to this straight edge (which is 
therefore much the steeper side) and at about one-third from 
the broader rounded end. 
Locality .—Chorhoti Pass, 16,000 feet. (No. 1754.) 
LAMELLIBRANCHIATA. 
It is not difficult to see that the mollusks of this little 
Silurian group indicate a deposit made in rather deep water. 
We may judge this from the numerous Brachiopoda, the 
small size of the Grasteropods, and the few Lamellibranchiate 
(or ordinary bivalve) shells. And these, too, appear to be 
deep-water forms—Nucula and its allies being characteristic of 
the deeper water;—and Ctenodonta is the Palaeozoic form of 
Nucula. 
CTENODONTA SINUOSA. 
Plate 2, fig. 20. 
C. modica, ovato-acuta, lente couvexa, concentrice imbricata, 
antice rotundata, postice acuta, sinuata, sinu conspicuo. Nates 
depress®, subcentrales. Line® concentric® regulares, imbricat®. 
Lat, 10 lino; alt. 6£. 
Our figure well expresses the contour of this rather large 
species, which measures at least ten lines broad, and six and a 
half lines high, not very convex, but most so in the middle of 
the disk—the beak being small and low, the anterior* side 
rounder and rather produced; the posterior long, pointed, with 
a very slightly arched cardinal slope, and a strongly-sinuated 
ventral line beyond the broadest part. The lines of growth 
are sharp, imbricating, and nearly equidistant ridges over 
those parts of the disk which are unworn, and therefore pro¬ 
bably extended in the same style over the whole. 
The surface is regularly and gently convex, except on the 
posterior slope, where a low ridge, bounded by shallow furrows. 
* Ctenodonta differs from Nucula cot only by having an external ligament, but by 
the posterior instead of the anterior side being largest, as in most other bivalves. In 
Nucula it is the reverse of this—the posterior side is the short one, and the ligament 
is internal. Leda is like Ctenodonta in shape. 
