SONNINIA— REVISION. 381 



undertake so laborious and apparently so wanton a task, of which the need was 

 not understood, — these were the causes why it was not done. But, when the 

 necessity was proved, the task was undertaken and carried to an end, though the 

 author stood thereby self-convicted of error. 



In connection with the following scheme for a classification of the Sowuiniee of 

 the Goncavum-zone, based upon the evidence of their various septa, it must be under- 

 stood that, in certain cases, Ammonites have septa which are unlike on the two 

 sides. Such cases may be classed as abnormalities, similar, though less in degree, 

 to such abnormalities as Ammonites with two different sides, or as d'Orbigny's 

 « Turrilites." In some cases these departures from bilateral symmetry may become 

 normal, as in the true Turrilites ; and then such a development, which might be 

 called "a persistent abnormality," would become of value in the matter of 

 classification. So far, in the Sonninidd, septal abnormalities have shown a certain 

 difference in the arrangement of the cells and of the lobules on the two sides, so 

 that the superior lateral lobe has, on one side, an isosceloid, on the other an 

 anisosceloid terminal lobule. It is, however, a case of far greater differences 

 than this being joined to great external similarity. The most striking example is 

 shown in PI. XCIV, in which figs. 1 and 3 illustrate two homceomorphous 

 fossils, while figs. 2 and 4 show septa which are wonderfully distinct, the superior 

 lateral lobe of fig. 4 being short, tridactyloid, symmetrical, with isosceloid, 

 equicellate terminal lobule ; that of fig. 2 being long, tetradactyloid, asymmetrical, 

 with terminal lobule intra-axial, inequicellate, and outer lobule deeply bipartite. 



The following scheme having been decided upon, it was considered advisable 

 not to continue the description of the remaining new species, but to treat the 

 whole series of Goncavum-zone Sonninise together, introducing the new species 

 where they fell in the scheme. It may be remarked that a bilaterally symmetrical 

 cruciform superior lateral lobe (see PI. CI, fig. 3) is taken as the archetype, from 

 which the others are considered to have developed ; but it is probable that, in the 

 phylogenetic history, this type of lobe was preceded by a superior lateral lobe 

 with ultra-axial, inequicellate, anisosceloid terminal lobule — the inner inter- 

 lobular cell the deeper, making the terminal lobule ecto-brachysceles, 1 like that 

 shown for Zurcheria parvispinata (PI. L, fig. 10) ; and that in passing from the 

 lobe with ultra-axial, inequicellate, ecto-brachysceles terminal lobule to the 



time required for each specimen would be very great. So far, in my experience, the tracing-off 

 processes are much the most expeditious — though in the case of complicated septa the time may be 

 reckoned by hours, — and the errors are not so much in the work of tracing and drawing, but in the first 

 marking out on the fossil. 



The above remarks are made in answer to certain queries as to a royal road to do suture-lines. 

 There is no royal road : it is a tedious process at best, and very trying to the eyesight. 



1 RpaxyaKeXris, short-legged ; Iktos, outside ; evbov, within. The shorter leg on the outer side or 

 on the inner side as the case may be (see p. 382). 



