7 ib YORKSHIRE TYPE AMMONITES— II May 



71. AMMONITES SUBTRIANGULARIS, Young & Bird 

 (Plates LXXIa, b) 



Original Description 

 [Young & Bird, 1822, pp. 250, 327.] 



" Figures 4 and 5, PL xn, represent fragments of two knobbed 

 ammonites that are rare and singular. No. 4, which is from the lower 

 part of the aluminous strata, is part of the outer whirl of an ammonite 

 that has measured about ten inches in diameter. Its aperture or section 

 is subtriangular, the back being not rounded, but with two acute angles, 

 and the sides sloping inwards, especially one side, which is more strongly 

 ribbed and knobbed than the other. The knobs on the plainest side, 

 in this specimen, consist chiefly of galena, a substance which we noticed 

 (p. 124) as occurring in the dogger, and which is not uncommon in the 

 petrifactions of the aluminous strata. — No. 5 is from the calcareous 

 sandstone below the oolite, on Silphoue moor. It has belonged to an 

 ammonite equally large, and with larger ribs and knobs, placed at greater 

 distances. The back is somewhat rounded, and that and the intervals 

 between the ribs are striated, whereas in No. 4 they are smooth. Unless 

 more appropriate names can be found, when entire specimens are obtained, 

 No. 4 may be termed a. subtriangular is, and No. 5 a. Silphouensis. 



" [P. 327] Plate xii. fig. 4. Ammonites subtriangularis. Lowest 

 shale." 



Remarks. 



Proportions : 210, 22, 26, 53 : Substeno-, subpachygyral, sub- 

 extremi-latumbilicate. 



Stages, conch, serpenticone ; periphery, 1 ; ornament, 5*. 



The fragment happens to be not equal sided : the left side (aperture 

 facing) is not so broad as the other. The left side is presumably what 

 Young & Bird call " the plainest side " ; but there is not much difference 

 in the degree of ornament. 



The fragment is the commencement of the body-chamber, part of 

 the last suture-line shewing 



Genus, Deroceras Hyatt ; family Deroceratidae. Geological position, 

 according to Simpson (1855, 66 ; 1884, 99), " the pyritous bed near 

 Bay-town " ; it is entered in L.L. y (1884, xx). 



Result 

 Deroceras subtriangulare, Young & Bird sp. 1822, Char- 

 mouthian, armatum zone, [Robin Hood's Bay, near Whitby]. 



