POTAMTDES (PTYCHOPOTAMIDES) TRTCINCTUS. tl3 



Dimensions. — L. 25 — 32 ram. B. 9 — 10 ram. 



Distribution. — Fossil: Icenian Crag, locally abundant. 



Remarks. — Potamides tricinctas, with its varieties, was regarded by Wood, and 

 in 1881 by Nyst 1 as the equivalent of the Murex tricinctus of Brocchi. It is now 

 included by M. Cossmann in the Ptychopotamides of Prof. Saoco, a sub-genus of 

 Potamides. 



The type form (figs. 23—25) is strong and solid ; it is fairly common in the 

 Waltonian zone, as at Oakley, becoming less so at the later horizons of the Red 

 Crag, while it disappeared from the Anglo-Belgian basin during the Icenian 

 period. 



The variety inomata, also a Red Crag form, resembles more or less nearlv the 

 variety subagranosa of Sacco, an Astian shell, as to which he remarks that occa- 

 sionally it has more than three spiral ridges, a feature which it shares with our 

 shell. This is shown in my figure 22. Wood's fig. M>. (op. cit.), which is very 

 good, illustrates the usual tricarinate form of this variety. The variety icenica is 

 smaller and the whorls are flattened, the base of the shell being subangular and 

 not rounded as in the type. In the Red Crag, the type form is fairly common, and 

 it is strong and solid ; in the Norwich Crag, on the other hand, where the var. 

 icenica is locally abundant, its appearance and texture are different, being thin and 

 fragile, so that it is difficult to obtain a perfect specimen of it, indicating I think 

 an alteration in the conditions under which this mollusc then lived. 



A similar difference may be noticed in the case of some other species, as in 

 Cardium edide, specimens of which in the Red Crag are large and strong with 

 coarse sculpture, while those from the Icenian Crag, as at Bramerton, are, as a 

 rule, small and fragile. A similarly small and fragile variety of Buccinum undatum 

 is also found at the latter place, the large and strong form of the Red Crag being, 

 so far as I know, absent. 



An explanation of these facts may possibly be found in the writings of Scandi- 

 navian and other authors. Dr. Munthe states, for example, that in the Baltic at 

 the present time the size and solidity of certain marine shells bear a fixed relation 

 to the salinity of the water; moreover, that during the existence of what is known 

 as the " Littorina Sea" when, owing to a subsidence of the land, the salinity was 

 greater than it is now, such molluscs, of which he specially mentions Cardium edule, 

 were larger and stronger than those living to-day in the same region. 2 



Sir Charles Lyell, moreover, dealing with the kitchen-middens of the Baltic, 

 states that the mollusca found in them are of the usual oceanic type, whereas those 

 now living in the same region only attain one third of their normal size, being 



1 The shell described by Nyst iu 1843 as Ceritluum tricinctum (Coq. foss. Terr. Tert. Belg., p. 539, 

 pi. xlii, fig. 7) was afterwards identified by him (op, cit., 1881, p. 80) with the C. Lamarckii of 

 Brongniart. 



2 Bull. G-eol. Inst. Univ. Upsala, vol. ii, pp. 3 et seq., 1894. 



