Vol. 70.] DEVELOPMENT OF TliAGOPHYLLOCEIiAS LOSCOMBI, 337 



II. Horizon - and History. 



Sowerby stated that his ammonite came from the ' Blue Lias.' 

 but gave no further details. The type-specimen, which is preserved 

 in the British Museum (Geological Department, Xo. 33425), shows 

 the bluish limestone-matrix, iridescent pearly layer, and general 

 mode of preservation of the specimen figured by Wright (1S80) 

 (B.M. C 2205) and stated to be a 



• very characteristic fossil of the " Green Ammonite Bed." Middle Lias near 

 Charmouth ' ; 



and further, numerous other specimens examined (Xos. 4-7 of 

 Table II, p. 343 are typical examples of these). There can be 

 no doubt that they come from the ' Lower ' and ' Upper Lime- 

 stones ' of Mr. Lang's classification. Tracfopliylloceras loscombi 

 occurs also in the intermediate ' Red Band ' ; but the fossils of 

 this bed are characterized by their different appearance. Oppel 

 (1S56) probably collected his specimens from the 'Lower Lime- 

 stone, ' for he states that Deroceras davoei occurred immediately 

 above, which is the case at Charmouth. All the specimens of this 

 rare fossil collected by Mr. Lang (1914, p. 328), at least, came 

 from this horizon : that is. from the ' Red Band ' or immediately 

 below. But Oppel erroneously considered Tr. numismale (Quen- 

 stedt), Avhich occurs in AViirttemberg in the Ja meson i Zone (and 

 apparently considerably below the davoei bed) as identical with 

 the later Tr. loscombi, an error which caused confusion for nearly 

 forty years, until Pompeckj (1S93) separated the two forms. Even 

 he, however, based their separation only on differences of dimen- 

 sions, umbilicus, and suture, and did not recognize their different 

 geological age. 



Numerous small ammonites preserved as internal casts in iron- 

 pyrites are found in the four clays of the Green Ammonite Beds. 

 These will be shown to be the young of Tr. loscombi, and from 

 them the ontogeny of this form has been worked out ; but they are 

 accompanied by a very few large specimens in a badly crushed 

 condition. The small casts are septate throughout, and in no ease 

 was a body-chamber discovered, hence they are not adult specimens 

 of an undeseribed form. Also, at a diameter of 2-5 mm. there was 

 no trace of a crenulation of the periphery that would indicate 

 affinity with Tr. ibex (Quenstedti; there is no resemblance to 

 young examples from Gloucestershire, which agree very well with 

 the descriptions of the young of Tr. ibex given by A. d'Orbigny 

 (1842 : as A. bobiayi) and by Pompeckj (1893, p. 22). 



In the Belemnite-Stone, which lies immediately below the base of 



it represented a family. Later authors have used the term formenreihe 

 for genera, as well as for only single genetic lines : that is, lineages. 



It is clear that ontogenetic evidence alone will enable us to trace the inter- 

 connexion of these developmental series, and thus approximate to a natural 

 (as distinct from the present more or less morphological) classification of 

 ammonites. 



2a2 



