3SGS.] jui)D — f>rhEr()N clav. 247 



a complete series between the somewhat common young tonii and the 

 rare adult. I have not yet been able to determine wliether the form 

 named A. trlsaJcosas by Phillips is really distinct. There can, liow- 

 ever, be little doubt that several Ammonites described by D'Orbig-ny 

 in his group of the L'lgati are really only forms of the British 

 species. 



A. hystriv, Phill. — This is a very well-marked and beautiful 

 species occurring in the Astierianus-beds, where, however, it appears 

 to be very rare. D'Orbigny records it from the Lower Neocomian 

 of South-eastern France. It is certainly quite disLinct from the A. 

 MantelU, a Lower-chalk form. 



A. marginatus, Phill., A. nucleus, Phill. — I was at one time in- 

 clined to consider these as the same species ; but the examination of 

 a large series of specimens in the Scarborough Museum shows that 

 the former, even in its younger stages, has the tubercles around its 

 umbilicus, of which the latter is destitute. They must therefore for 

 the present be kept distinct ; their adult forms are, so far as I am 

 aware, altogether unknown. The species referred by Eomer to A. 

 nucleus is certainly quite distinct from the British form, and appears 

 to belong to a JSTeocomian species not yet, I believe, found at Speeton. 



Ammonites angulicostatus, D'Orb. — This very interesting species 

 appears to form a connecting link between the genera Ammonites 

 and Ancyloceras. In the young form the whorls are in contact, but 

 with advancing age the outer whorl becomes slightly detached from 

 the others. By Quenstedt this species was regarded as belonging to 

 Crioceras. It is well figured and described by Pictet and De Loriol, 

 ' Terrain Neocomien des Yoirons ' (Invertebres), p. 23, plate 4. 



Several other species of Ammonites, some of which appear to be 

 new, also occur in the Neocomian beds at Speeton. 



The various coronated forms of Ammonites which occur at Speeton, 

 and were called by Mr. Bean A. cavatus, A. cjuadri/idus, &c., are 

 certainly identical with certain continental Portlandian species, as 

 already noticed by Mr. Leckenby. The cabinet of that gentleman 

 contains the most splendid materials for their study. 1 cannot help 

 thinking that they will all prove to be varieties of one species, thougli 

 for the present I have kept them distinct, and called them by the 

 names under which they are known on the continent. 



Ancyloceras. — Great as are the difficulties attending the study of 

 the Speeton Ammonites, they are light in comparison with those 

 which meet us in working out the species of the genus Ancyloceras. 

 In examining a large series of forms belonging to this group we are 

 struck by their enormous powers of variation. Thus a specimen 

 which has grown to a certain stage with simple ribs, and without 

 any appearance of spines, will suddenly exhibit a most extraordinary 

 development of those appendages ; and in the same way a most stri- 

 kingly spinous species will as suddenly become smooth. Now, in the 

 large majority of instances, we find at Speeton small and detached 

 fragments of a single whorl only, which for the purposes of identifi- 

 cation and description are absolutely worthless, and which, if so made 

 use of, can only be sources of error. For these reasons I think that 



VOL. XXIV. PAKT I. T 



