BIVALVIA. 301 
TEREDO NAVALIS. G. Johnston. Mag. Nat. Hist., vol. ii, p. 23, fig. 7, 6, 1829. 
_ -- Dekay. Nat. Hist. New York, p. 34, fig. 325. 
—  Brueurerr. Delle. Chiaje. Mem., vol. iv, pl. 54, figs. 9—12. 
— —_ Phil. En. Moll. Sic., vol. i, p. 2; and vol. ii, p. 3. 
Spec. Char. Testa tumidd, convexa ; angularibus striatis ; utraque hiante, hiatu postico 
magno angulato ; antico ovato. 
Shell tumid, convex, subspherical ; marked externally with angular strie; gaping 
widely on both sides; posterior with a large angular opening; anterior ovate. 
Diameter, 4 inch. 
Locality. Cor. Crag, Sutton, Ramsholt. 
Red Crag, Sutton. Recent, Norway, North America, Britain, 
Mediterranean, and Black Sea. 
A single valve, in my cabinet, is all that I have seen from the Crag, and this I 
believe to be zavalis of Mont. Fig. 12, c, represents a fragment of the posterior por- 
tion of the tube, probably of this species, showing the camerated structure; but there 
is no appearance of longitudinal division. It is, however, too imperfect for any safe 
reliance in that respect.* Fragments of tubes are not unfrequently met with in the 
Red, as well as in the Coralline Crag, perhaps belonging to the same animal. They 
are thick and strong, with a diameter varying from }th to 4th of an inch. 
The valves of the different species of this genus preserve a great similarity, and it 
is said they can only be determined specifically by the pallets or styles at the posterior 
orifice, and as these have not, that I am aware of, been found in the Crag, the present 
appropriation, if the above be true, may be considered doubtful. 
This portion of the Monograph contains the descriptions of forty-six species of 
Bivalves from the Lower or Coralline Crag Formation. Of this number, there are 
thirty-two identical with forms still in existence. From the Red Crag are described 
thirty-nine, out of which I have considered twenty-five as living species. Hence it 
will be seen that the former gives an amount of identification as high as 69 per cent., 
with a consequent extinction of 31; while the latter (Red Crag) has an amount of 
extinction as high as 36 per cent., giving, by this estimation, a greater antiquity to the 
higher or newer of the two Formations; and even presuming a better examination 
might bring the Red Crag on to an equality with the Coralline, the two Formations 
do not appear by this test to be capable of separation. 
May it not be asked whether it is necessary, in order to ascertain the age of a 
Tertiary Formation by the per centage system, and to assign it to one of the tripartite 
or quadrupartite divisions of the Cainozoic series, we are to take the whole of the 
* This part of the tube, according to Montague, affords little or no assistance in the determination of 
the species ; the number of these camerations or partitions vary from four to as many as twenty; indeed, 
in one specimen he examined, he counted not less than twenty-nine. 
