310 MR. L. F. SPAT1I ON THE [Dec. I9I4> 



not only the external saddle and first lateral lobe, bnt also the first 

 lateral saddle already dividing. A further saddle has appeared 

 at the umbilical end, and the internal suture now consists of a 

 very dee]) and bifid dorsal lobe and two long and narrow saddles. 



Between the diameters of 4 and 8 mm. (text-rigs. lf-lk, p. 341) 

 the suture consists of five saddles ; and especially noteworthy is the 

 liionoplvyllic character of the large terminal leaflets. The mono- 

 phyllic ending of the external saddle is characteristic, even in the 

 adult suture ; and the terminal leaflet is larger than the two leaflets 

 below up to a diameter of 14 mm. At that diameter the first 

 lateral saddle assumes the peculiar, subdiphyllic character which it 

 shows throughout the adult stage. The internal lower leaflet 

 increases in size, and grows forwards until it nearly rivals in height 

 the terminal leaflet, and. with it, forms the characteristic claw-like 

 ending of the first lateral saddle, which is as distinct in Tragophyl- 

 loceras ibex and in Tr. numismale, as it is in Tr. loscombi. The 

 four auxiliary lobes found in the adult suture are developed already 

 at a diameter of 20 mm. They decrease regularly towards the 

 umbilicus, and keep just above the normal : that is, the line con- 

 necting the deepest part of the ventral lobe with the centre of 

 the shell. Throughout the development the first lateral saddle is 

 higher than the external saddle, and the first lateral lobe is deeper 

 than the ventral lobe from a very early stage. 



With regard to variation in suture-line, it should be mentioned 

 that a specimen (figs. 3 & 4, PI. XLIX) from a higher zone already 

 shows a stage corresponding to text-fig. 1 d (p. 341) at a diameter of 

 1*8 mm., and the stage If (reached by the t}^pical forms only at 

 the diameter of 4 mm.) at 3 mm. On the other hand, a larger 

 specimen from the horizon of the Upper Limestone shows, at the 

 diameter of 17 mm., a suture which is essentially that of text- 

 fig. 1 m (p. 341). 



Before the allied forms are considered, the description of 

 Tragophylloceras loscombi given by Sowerby ought to be extended, 

 in consideration of the developmental evidence. A formal de- 

 scription might run thus : — 



Shell compressed, subextremiperplatygyral (height = 52 to 55 

 per cent, of the diameter), with slightly convex sides that merge 

 without edge into a rounded periphery. The sectional view thus 

 shows an elongated oval ; the greatest thickness occurs at one- 

 third of the height (23 to 26 per cent, of the diameter, therefore 

 subleptogyral) ; the umbilicus measures 10 to 14 per cent, of the 

 diameter (angustumbilicate), and has a vertical wall with a rounded 

 edge ; the involution amounts to five-sixths of the height. The inner 

 whorls are first depressed, with no inclusion, and at a diameter 

 of about 2 mm., circular with slight inclusion and an open 

 umbilicus of approximately 35 per cent. Between the diameters 

 of 5 and 20 mm. the shells are but slightly thicker and more 

 widely umbilicated than the adult shells : at a diameter of 25 mm., 

 however, the characters of the latter are typically developed. 



