64 



simple, and nearly always eqiiidistant, except on the ventral 

 surface where they beconie rather closer, leaving very regular 

 valleys between them, usually equal in width to that of three 

 keels ; the -whole of the keels and valleys are crossed by flno, 

 regular, oblique, equidistant, continuous, longitudinal strite. 

 Length (average) one and a half inches. 



Obs. — The spiral keels are remarkably regular, and simple in 

 structure, at least twenty and perhaps more traversing the 

 body whorl ; they retain their size as a rule, but instances 

 have occurred where they are alternately larger and smaller. 

 The oblique transverse striae are uniform in size throughout 

 their course, and do not in themselves enlarge on crossing the 

 keels, but the mere intersection of the two does give rise to a 

 slight nodular appearance. The spaces thus enclosed between 

 the two series of lines are oblong. 



It cannot be denied that both this shell and those usually 

 referred to Cydonema by authors, materially differ in appearance 

 from Hall's types of the genus. If a Cycloiie/ina, it is a very 

 large form, but at first sight it appeared to me referable to the 

 allied genus Oriostoma, M. Chalmas, as defined by Dr. G. 

 Lindstrom,* but the lengthened spire gives rise to a marked 

 difference in the appearance of the two shells, nor does 

 C. ? australis appear to have that laxity of coil seen in species 

 of Oriostoma. 



C. ? australis is allied to the shell termed C. cai'inatum, var. 

 multicarhiatum, by Lindstrom,! but which I should have taken 

 to be a species distinct from the other Gotland forms. Our 

 species is a larger shell, -with coarser and more numeroiis 

 revolving keels on the whorls. The operculum should be 

 sought for, as this will probably afford a means of accurately 

 determining the genus. 



Genus Oriostoma, M. Chalmas, 1870. 

 (Journ. Conch., Paris, xxiv., p. 103.) 



Oriostoma northi, sp. nov. 



(PL ix.jFigs. 6 and 7.) 



8p. Char. — Shell discoid or ammonitiform, biconcave, non- 

 alate ; back un symmetrically convex ; spire very short, depressed 

 below the plane of the body whorl ; whorls about six, all ex- 

 posed ; body whorl generally convex on the upper su.rface, 

 sloping downwards and inwards, bounded exteriorly by a 

 strong spiral keel, from whence the surface gradually slopes 

 away outwards to foi'm the unsymmetrical convex back ; the 



* Silurian Gastropoda and Pteropoda of Gotland {K. Svenska Vet. Akad- 

 Handlingar, 1884, xix.. No. 6), p. 156. 

 fLoc. cit., p. 179, t. 18, f. 31-32. 



