394 W. IT. Sudleston — Gasteropoda of the Portland Eoclcs. 



Neritoma is first observed in the Middle Kimmeridge of N.W. 

 Germany, next in the Lower Portlandian of Boulogne, and finally 

 in certain restricted beds of our own Portland iiocks, but always 

 on a higher horizon in its journey westwards. 



There is something very instructive in this, and Neritoma is so 

 marked a form that there can be no mistake about it ; but a similar 

 lesson is taught by other fossils. If, for instance, we wisli to search 

 in N.W. Germany for the representatives of many of our Portlandian 

 Gasteropods, to what horizon ought we to turn ? Not certainly to 

 the Upper Portlandian of that country, for it does not contain a 

 single Gasteropod.' The Lower Portlandian of N.W. Germany 

 (zone of Am. gigas) has a larger and more varied fauna, but we 

 must go as low as the Pteroceras schichten, that is to say, below the 

 equivalents of the mass of the Kimmeridge Clay of England, which 

 is mainly Virgulian or higher, before we recognize anything like an 

 univalve facies corresjDonding to that of our own Portland Rocks. 



10. Nerita tkansveksa, Von Seebach, 1864. Plate XI. Fig. 9. 



Neritn transversa, Von Seebacli, 1864, Der. Han. Jura, p. 131, pi. vii. figs. !«, h. 

 Idem. Idem. 1866, De Lor. et Pell., Port, de Boulog. p. 33, pi. 



iii. figs. 22-24. 

 Ihid. varietas minor. Lor. et Pell., 1874, Jurass. sup. de Boulogne, p. 105, pi. ix. 



figs. 16, 17. 



This interesting form, which is too well known to need descrip- 

 tion, is another connecting link between our Portland Eocks and 

 the Middle Kimmeridge of Hanover. It occurs in some abundance 

 in the Lower Portlandian of Boulogne, together with the variety 

 minor, which alone occurs in the Upper Portlandian of that district. 

 De Loriol speaks of it as a perfectly typical Nerite living under 

 marine conditions, but one may read the fact the other way, as 

 showing the estuarine character of beds where such shells occur. 



In England its occurrence was first noted by Mr. Blake in the 

 Cyrena-heds of the Vale of Wardour, where it is met with sparingly. 

 No instance of its occurrence in the more truly marine beds has yet 

 come to my knowledge. 



11. AcTiEONiNA siGNUM, sp.n. Plate XL Figs. 7a, 7b, 7c. 



Description. — Specimen from the Cyrena-heds, of Chilmark Quarry. 



Length 20 millimetres. 



Width 16-5 „ 



Diam. of last Avhorl in relation to its length .... 0-78. 



Shell tumid, not much longer than wide, subcylindrical, attenuated 

 anterioT-ly. The spire, which scarcely projects, consists of five 

 or six whorls, one within the other. These are faintly convex, 



1 Subjoined is a complete list, according to Struckmann, of the fossils of the 

 Upper Portlandian of Hanover. Fecten concentricus, Dunk, and Koch, Perna 

 Bouchardi, Oppel, Modiola Uthodomus, Dunk, and Koch, Trigonia variegata, Creduer, 

 Cardimn Jjufreynoicmn, Buvig., Cyprina Brongniarti, Room., Cynna rugosa, Lor. 

 (Sow.), Cyrma nuculcefurmis, Ecem., Gyclas Brongniarti, Diuik., Corbula Mosensis, 

 Buvig., Corbula alata, Sow. 



