118b YORKSHIRE TYPE AMMONITES— II Dec. 



118. AMMONITES DISSIMILIS, Brown. 

 (Plates CXVIII a, b) 



Original Description 

 [T. Brown, 1849, PP- 2 4°> 2 59-J 



" ig. Ammonites dissimilis. — The Dissimilar Ammonite ; pi. xx*, 

 figs. 11, 12, 13. 



" Inflated with rapidly increasing volutions, the smaller ones entirely 

 concealed ; largely umbilicate ; crossed by broad furrows, and flattened 

 curved ribs passing over the thick back, with a few elongated nodules 

 on the inner edges of the volutions ; aperture wide and sub-orbicular. 



" The Calcareous Grit, Scarborough. 



" In the Manchester Museum. 



" [P. 259] Plate xx*. . . 11, Ammonites dissimilis, var. p. 246, 

 12, 13 Ammonites dissimilis, p. 246." 



Remarks 



Two specimens were figured by Brown who appears to have employed 

 a MS. name originally given by Williamson, see Note below and remarks 

 on Am. rotifer, No. 113. Two specimens are in the Manchester Museum: 

 one (PL CXVIIIa) may be accepted as original of Brown's PI. xx* f. 12 (13 ?) 

 and would thus be his Holotype, the other (PI. CXVIIIb) is by its label 

 Williamson's Chirotype (though both may have been so), and just possibly 

 original of Brown's Am. dissimilis var., PL xx* f. 11 : it has the long 

 primaries and curving secondaries of Brown's fig., an umbilicus with 

 similar amount of matrix and a rough piece of matrix at end of whorl. 

 It can however only be considered original of Brown's fig. if his delineation 

 was an enlarged one. 



Proportions: }^ ^ ^| ? ^ ^3 | Holotype. 



Perplaty-, subextremipachygyral ; subangustumbilicate. 



Globescent, galeatiform, § occluded ; periphery subangulate at 

 beginning of last whorl, losing this and becoming rounded at end. Inner 

 margin slightly rounded at beginning of last whorl ; strong, very steep, 

 subplanate at end. Ribs with somewhat short (about J- of side), some- 

 what tumid primaries, bifurcating into slightly curved secondaries (with 

 occasional intercalation — failure of primary) crossing periphery at angle 

 which widens out as globosity increases : all rib-characters becoming 

 obsolete, end of whorl probably quite smooth : another whole whorl is 

 indicated by remains of contact-line of inner margin. Suture-line, Li 

 longi-, angustilobate ; L2 quite short, rather broad. 



Genus, Eboraciceras, nov. (p. xiv) ; family, Cadoceratidse, Hyatt, 

 1900. This species appears to be the same as Am. sutherlandice , d'Orbigny, 

 clxxvi, but Am. Sutherland i a , Murch-Sow., is a more advanced (more 

 gerontic) form, beginning to expand umbilicus and decrease thickness, 

 see p. 118c. 



Geological position : Matrix bluish, marly, calcareous with oolitic 

 grains somewhat close in places — Vertumnus zone, Q.J.G.S. LXIX, 1913, 

 P- 153. 



Result 



Eboraciceras dissimile, Brown sp. 1849, [Divesian, vertumnus 

 zone], Scarborough, Yorkshire. 



