Vol. 70.] DEVELOPMENT OF TRAGOPHYLLOCEIiAS LOSCOMBI. 349 



in my own collection, it appears that the stage shown in fig. 1 k 

 (p. 341), reached by Tr. losconibi at the diameter of 8 mm., is not 

 attained by Tr. ibex until between 10 and 18 mm. Constrictions 

 and suture, therefore, point, here also, to a more primitive condition 

 than in Tr. loscombi. The adult ibex suture is characterized by 

 slender and lon^ terminal leaflets on the saddles, and bv the raisins 

 of the auxiliaries above the normal, both of which characters were 

 observed in the slightly-crenulated form from the Belemnite Stone 

 (fig. 2 e), and in another, similar, specimen, not found in place, 1 

 which shows these slender leaflets already present at a diameter of 

 15 mm. 



Whether, now, we accept Pompeckj's developmental series numis- 

 male-wechsleri-ibex, or the more probable lineages put forward by 

 Futterer (namely, numis male- wechsleri on the one hand, and numis- 

 male-ibex on the other), it seems that none of them leads to 

 Tr. loscombi. In the latter form, if the ornament tends at all to 

 thicken on the periphery, it does so on body-chambers only, and 

 on these at various horizons up to margaritatws. The section is 

 uniformly oval between 5 and 20 mm., at which stage in the earlier 

 Tr. ibex it is at first high and sharpened peripherally, and then 

 rectangular ; finally, the suture-line of Tr. loscombi, still at a 

 diameter of 14 mm., shows a large terminal leaflet of the external 

 saddle with two smaller ones below. We must, therefore, look to 

 a iunnisinale-\We form for the ancestor of Tr. loscombi, and not to 

 the stronglv ornamented ammonites of the Ibex Zone. 2 



With regard to the curious distribution of the forms belonging; 

 to the genus Trogoplnjlloceras in time and space, it may be 

 suggested that the original, probably Mediterranean, ancestor 

 (whose aflinities with various Lower Liassic and Triassic families 

 I shall attempt to trace in the following pages), after migrating 

 into the Swabian Carixian Sea, did not thrive 3 until it took on 

 marked ornamentation, when it spread as far north and Avest as 

 England and Xormandy. The normal, compressed and involute 

 development (Tragopliylloceras loscombi) followed the migration, 



1 From the preservation and general appearance of this specimen, as well as 

 from the circumstances in which it was collected. Mr. Lang thinks it likely 

 that it came from a foot or so below the Belemnite-Stone, where he found 

 Acanthopleuroceras ellipticum (Sow.). 



- It is interesting to note here that a body-chamber fragment of a specimen 

 of Tragophylloceras ibex measuring about 130 mm. in diameter, from Hewlett's 

 brickyard, near Cheltenham (B.M. C 8876). has the lineate ornament of Tr. 

 >>mbi on the last four-fifths of its half- whorl and is only distinguished by a 

 tabidate periphery and a wide umbilicus. At the posterior end of the body- 

 chamber the typical ibex periphery can still, however, be seen ; the last suture 

 only is preserved, but unfortunately worn down and therefore not traceable. 



On the evidence of this specimen one might be tempted to consider the 

 later, involute Tr. loscombi as a descendant of Tr. ibex, the omission of the 

 costate-crenulate stages being explained by saltative palingenesis. It must 

 be remembered, however, that the suture-line in that case would presumably 

 be of a different type in the later form. But the specimen can be regarded as 

 proof of the close relationship, and more or less parallel development, of the 

 ibex- and loscomb 1 -branches in the genus Tro igoph ylloceras. 



3 Tragophylloceras numismale is a rare fossil. 



