BIVALVIA. 301 



TEREDO NAVALIS. G. Johnston. Mag. Nat. Hist., vol. ii, p. 23, fig. 7, b, 1829. 



Dekay. Nat. Hist. New York, p. 34, fig. 325. 

 BRUGUIERI. Delle. Chiaje. Mem., vol. iv, pi. 54, figs. 9 12. 



Phil. En. Moll. Sic., vol. i, p. 2 ; and vol. ii, p. 3.J 



Spec. Char. Testa tumidd, convexd ; angularibus striatis ; utraque hiante, hiatu postico 

 magno angulato ; antico ovato. 



Shell tumid, convex, subspherical ; marked externally with angular striae ; gaping 

 widely on both sides ; posterior with a large angular opening ; anterior ovate. 



Diameter, J 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 navalis of Mont. Fig. 1 2, 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 |th 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. 



