336 ME. L. F. SPATH OX THE [Dec. I914, 



16. On tlie Development of Tragophylloceras loscombi 

 {J. Sowerby). By Leoxaed Fea> t k Spath, B.Sc, F.Gr.S. 

 (Read May 27th, 1914.) 



[Plates XLVIII-L.] 



Contents. 



Page 

 I. Introduction 336 



II. Horizon and History 337 



HI. Ontogeny 339 



IV. Phylogeny 350 



V. Bibliography .' 359 



I. Lsteoductiox. 



As is well known, few localities are so productive of well-preserved 

 Liassic ammonites as the cliffs at I/vrne Regis and Charmouth. 

 They have furnished a considerahle proportion of the types of 

 Lower Liassic ammonites created nearlv a centurv as^o in ' The 

 Mineral Conchology of Great Britain,' and one of these types, 

 fortunately one of those the figures of which were good enough 

 to allow of correct interpretation by later authors, is Ammonites 

 loscombi J. Sowerby (1817). 1 During a prolonged investigation 

 of the Liassic succession at Charmouth, Mr. W. D. Lang (1913) 

 has collected abundant fossil material with particular reference 

 to its exact stratigraphical horizon, and I am indebted to him 

 for permission to study the ammonites. In this material, Trago- 

 phylloceras loscombi is represented by hundreds of specimens 

 (chiefly young), and a study of the ontogeny - of that interesting 

 ammonite forms the basis of the present paper. The bearing of 

 certain important facts brought out in this study upon the 

 phylogeny of the genus TragopJiylloceras, and the speculations 

 on the connexion of the latter with allied lineages. 2 will prove, it is 

 hoped, of general interest. 



1 For full reference see the list of works at the end, § V, pp. 359-60. 



2 The term lineage is here employed to denote a single genetic line or 

 series, that is, generally, a genus : although a genus may comprise several 

 lineages, and even their lateral branches, so long as these lineages are offshoots 

 of the same stock. Of course, it would be best to distinguish by a different 

 name every series recognized by ontogenetic evidence as a distinct lineage. 

 But I have not abandoned the customary use of the term g enus for reasons 

 of convenience, since an extraordinarily large number of new lineages would 

 have to be created for, often, only one or two forms. 



It will be noted that the term lineage was originally suggested by J. F. 

 Blake (1892) as a substitute for genus, although he uses the term himself 

 for the family Arietidae as well as for the sequence C celoc eras- Pel tor eras - 

 Aspidoceras. The term f ormenreihe, translated by Waagen (1875) himself 

 as ' developmental series,' in its original meaning (1869) included a number 

 of lineages and genera diverging from a radical form, and therefore its con- 

 notation was more extensive than either lineage or genus; in other words, 



