Vol. 70.] DEVELOPMENT OF TBAGOPHYLLOCEHAS LOSCOMB1. 347 



The innermost two and a half to three whorls bear from five 

 to twelve constrictions persisting not later than the diameter of 

 3*5 mm. 



The shell is ornamented with delicate costae, of which there are 

 forty on the last half-whorl of the type-specimen. They describe 

 a sigmoidal curve, having- a convexity forward at the inner third 

 of the side and crossing' the periphery in a second convex curve 

 forward. Occasionally in older shells (on the body-chamber) the 

 radii tend to thicken along- the rounded ventral region to form 

 faint scales. Length of body-chamber = probably nearly three- 

 quarters of a whorl ; mouth-border and aptychus unknown. 



The suture-line normally presents four auxiliary lobes. The 

 external saddle is lower than the first lateral saddle ; the median 

 saddle of the ventral lobe is comparatively high, and the first 

 lateral lobe is deep. The external saddle is monophyllic through- 

 out the development, as are all the other saddles except the first 

 lateral saddle, which assumes a subdiphyllic appearance in the 

 adult. The terminal leaflets of the three principal saddles are 

 lanceolate, spoon-shaped in the adult ; the remainder, like the 

 young saddle-endings, truly phylloid. 



Allied Species. 



Mr. Buckman (1910) has redescribed and figured Simpson's 

 Ammonites ambiguus, which came from Robin Hood's Bay, 

 presumably from the Valdani Zone. 1 This ammonite differs very 

 little from Tragophylloceras loseombi? Its umbilicus is 15 per 

 cent, of the diameter, instead of the usual 10 to 14 per cent. ; and 

 its thickness is probably 29, not 23 to 26 per cent. Its whorl- 

 section may also be a little more trigonal in shape. Unless found 

 to differ in suture-line, however, this form would hardly he regarded 

 as more than a variety of (mutatio ascendent ad) Tr. loseombi? 



The other ammonites included in the genus Tragophylloceras 

 can be divided into two groups : namely, that of Tr. numismale and 

 that of Tr. ibex. The former includes, besides the trigonal-whorled, 

 striate Tr. numismale (Quenstedt), its crenulate, compressed 

 development Tr. typicum S. S. Buckman ; the stout- whorled Tr. 

 elteni (Pompeckj), and the densicostate Tr. intracrustatum 

 (Quenstedt) ; the similar but more crenulate Tr. wechsleri (Oppel), 

 and the sparsicostate Tr. paucicostatum (Pomp.). A number of 

 forms such as Tragophylloceras n. sp. (= A. ibex-heterophyllus 

 Quenst. 1885) ; Futterer's (1891) fig. 6a of pi. viii ; and Wright's 

 (1880) fig's. 1-3 of pi. xxxix, are transitional to the typical, 



1 At first, also with a query, recorded from the Capricoriws Zone. 



- Tate & Blake (1876. p. 296) considered it to be a synonym of Tr. loscombi 

 (J. Sowerby). 



: ' Ammonites nanus Simpson and A. huntoni Simpson are, according to 

 information kindly forwarded by Mr. Buckman. probably young forms of 

 Tragophylloceras. Similarly, ..4. robinsoni Simpson may belong here, to judge 

 by two specimens in the British Museum (X. H. 37987) bearing Bean's 

 original label. 



