AMMONITES. 41 



A. testa discoided, costatd, tuberculatd : anfractihim angulatis : costis simplicibus, recti- 

 usculis, incequalihus ; ad dorsum suhcontinuis vel ohsoletis, hituherculatis ; ad laterd 1, 2, 

 vel 3 tuberculatis : dorso angulato, medio depresso : umhiUco parviusculo : aperturd r/iom- 

 boided : septorum lobis later alihus,primo incsqualiter bifida, cateris trijidis. 



Var. A. Anfractibus tumidis : aperturd octangulari ; costis rectis, majoribus 8 tubercu- 

 latis, minoribus 6 tuberculatis; tub er cutis spinosis. 



Var. B. Anfractibus compressis : aperturd angulato-ovali : costis subjlexuosis : tuberculis 

 dorsalibus conspicuis, ccBteris obsoletis. 



Shell discoidal, with twenty to twenty-five tuberculated ribs, alternately long and 

 short : whorls angular, depressed or tumid : ribs simple and usually straight, the longer 

 ones commencing at a tubercle at the edge of the umbilicus, the shorter below the middle 

 of the whorl, and all continuing across the back, with a depression in the middle of the 

 back : tubercles varying from four to eight rows, of which the umbilical and dorsal rows 

 are always present, the lateral rows being either well or faintly marked, or altogether 

 absent : back depressed in the middle, with a row of tubercles on each side : umbilicus 

 small and deep, allowing half the inner whorls to be seen : aperture rhomboidal : septa 

 with three lateral lobes, the first unequally bifid, the others trifid. 



Var. A. Whorls tumid : aperture octangular : ribs straight, with eight rows of large 

 tubercles. 



Far. B. Whorls flattened : aperture nearly oval : ribs slightly flexuous : dorsal tubercles 

 distinct, the others nearly obsolete. 



Diameter, 3 inches; height of last whorl, 1| to 1^ inch; width of aperture, 1^ to 1| 

 inch, according to the variety. 



Found in the Lower Chalk and the Upper Green Sand of the South of England, and 

 in beds of the same age throughout France. 



The distinction between this species and A. navicularis will be found after the descrip- 

 tion of that species. 



28. Ammonites Renauxianus, B'Orbigny. Plate XIX, fig. 2. 



Ammonites Renauxianus, IfOrhigny. Paleont. Fran?. Terr. Cret., t. xxvii. 



A. testa discoided, compressd, tuberculato-costatd : costis bi- vel tri-fidis, jlexuosis, bi- 

 tuberculatis, ad dorsum sub-interruptis : dorso bituberculato ; juniore transversim sub-costato ; 

 adulto sub-carinato : umbilico mediocri : aperturd truncatO'eUipticd. 



Shell discoidal, compressed, with few whorls, ribbed and tuberculated : ribs, about 

 fifteen, bifid or trifid, each springing from a large tubercle near the edge of the umbilicus, 

 and dividing into two or three flexuous branches, each of which bears another tubercle at 

 the edge of the back : in young shells the ribs are faintly continued across the back ; but 



