Vol. 59.] TWO TOARCIAN AMMONITES. 461 



or a species which is the morphic equivalent thereof. The inner 

 whorls of the specimen figured are a good morphic representation 

 of M. Birchi. 



Thus in the phylogenetic history of Deroceras would be found 

 the stages smooth, costate, unituberculate, bituberculate, unituber- 

 culate ; though ontogeny of the usual D. armatum may often show 

 no more than smooth, costate, unituberculate — the bituberculate 

 stage having, as it were, been squeezed out by tachygenesis between 

 its pre- and post-unituberculate stages. The smooth, costate, and 

 unituberculate stages which lead up to the full bituberculate deve- 

 lopment of Microderoceras Birchi are well shown in the ontogeny 

 of that species. What two of the stages would have been like 

 as separate species may be learnt from Ammonites planicosta and 

 Am. cciphns, which show the costate and unituberculate stages 

 respectively. These species cannot be the actual ancestors of 

 Microderoceras Birchi, because they are later in date ; but they are 

 the morphic equivalents of those ancestors : they show what those 

 ancestors would have been. They are the anagenetic stages. 



These anagenetic stages show that, by analogy, there is good 

 reason to suppose that species having similar features will be 

 found leading up to Chartronia binodata ; or, at any rate, that that 

 species passed through such stages to arrive at its present condition. 

 The cases of degenerative development from bituberculate to costate 

 stages, which have been cited above, indicate that a costate species 

 like this costigera may be placed as a catagenetic development from 

 Chartronia binodata, and be quite in accordance with the develop- 

 ment shown in other series. 



Note on Technical Teems. 



Concerning the technical terms here used the reader is referred 

 to ' Monogr. Inf. Ool. Amm.' Suppl. i. (Pal. Soc). But, in order to 

 secure so far as possible an uniform value for these terms, it has 

 been found advisable to use a more definite standard. This is fur- 

 nished by the radius, that is, the length from the centre to the 

 periphery. This being taken as 100, the percentage of other dimen- 

 sions may be approximately stated as follows : — 



To 17 per cent Perstenogyral. 



Ferleptogyral. 



Perangustumbilicate. 

 From 17 to 34 per cent.... Stenogyral. 



Leptogyral. 



Angustumbilicate. 

 From 34 to 50 per cent. . . . Substenogyral. 



Subleptogyral. 



Subangustumbilicate. 

 From 50 to 66 per cent Subplatygyral. 



Subpachygyral. 



Sublatumbilicate. 

 From 66 to 83 per cent Platygyral. 



Pachygyral. 



Latumbilicate. 

 From 83 to 100 per cent. .. Perplatygyral. 



Perpachygyral. 



Perlatumbilicate. 

 a J. G. S. No. 236. 2 l 



