442 A. E. Trueman — The genus Polymorphites. 



II. — Observations on the genus Polymorphites. 

 By A. E. Trueman, M.Sc, University College, Nottingham. 



IN 1843 Simpson published a description of a Yorkshire Ammonite 

 previously named Ammonites trivialis in Bean's manuscript. 1 

 A few years later Quenstedt described a number of small continental 

 Ammonites of similar character as A. polymor phus, which he further 

 divided into the varieties lineatus, costatus, interrupts, mixtus, and 

 quadratics. The specimens he then figured must be taken as his 

 types 2 ; in a later work 3 he showed many more examples which he 

 referred to the same species; all these have small, evolute shells, but 

 few other common characters, so that the name polymorphus was 

 fully justified. For example, the illustrations given as Ammonites 

 polymorphus mixtus include forms with round or square whorls, with 

 nearly straight or with curved ribs, which may pass across the venter 

 with or without a forward bend, or may be suppressed entirely on the 

 venter. The specimens figured probably include young examples of 

 several different genera. 



Sutner and Haug 4 systematized our knowledge of these fossils and 

 established the genus Polymorphites , while Simpson's specimens have 

 recently been figured and described by Buckman 5 as P. trivialis, 

 Bean-Simps. ; P. mixtus, Qu. ; P. Jupiter, d'Orb. The following 

 notes are mainly concerned with these three species. Most of the 

 specimens used in this work were pyritized casts obtained from the 

 Lower Lias clays on the tunnel heaps at Old Dalby in North Leicester- 

 shire ; accoi'dingly their precise horizons cannot be given. With 

 them there were also found P. caprarius, Qu., P. quadratus, Qu., and 

 other fossils of the valdani-zone. For permission to make use of 

 the abundant material in the collections at University College, 

 Nottingham, and for much help in the work, I desire to express 

 my thanks to Professor H. H. Swinnerton. Mr. S. S. Buckman has 

 also kindly given valuable suggestions. 



Comparison of the Species. 



Polymorphites trivialis is a costate form ; occasionally a strongly 

 ribbed example may show a row of elongate tubercles on the external 

 margin. The venter may be somewhat angular, thus resembling 

 P. caprarius, but frequently it is more or less rounded in the adult, 

 when the form may approach P. quadratus. 



P. mixtus is less strongly ornamented ; in the development of this 

 species the shell is at first smooth, then striate, and in the adult 

 subcostate to costate. In this respect it is intermediate between 

 P. trivialis and P. jupiter, which is an evolute simple form, smooth 

 until a diameter of about 4 mm. is attained, when slightly curved 

 stria? are developed. The developments of these forms are compared 



1 M. Simpson, Monog. Lias Ammonites, London, 1S43. 



2 F. A. Quenstedt, Die Cephalopoden, 1S49, pp. S6-7, tab. 4. 

 2 F. A. Quenstedt, Die Ammon. de Swab. Jura, 18S5. 



4 E. Haug, "Ueber die Polymorphidse " : Neues Jahrb. f. Min., 1887. 



5 S. S. Buckman, Yorkshire Type Ammonites, vol. i, No. 53, 1912. 



