462 MR. S. S. ETJCKMAN ON [Nov. I903, 



When the dimensions exceed 100 per cent, they may be denoted 

 by affixing the word extreme. Thus certain species might be 

 extreme-pachygyral, with further modifications by per, or sub, 

 when necessary. 



There is a certain arbitrariness about this method, as when a 

 slight difference on each side of a dividing-line gives a different 

 designation, while more difference (if falling towards beginning and 

 end of a division) does not, although such may be necessary in 

 specific distinction. 



When a dimension falls on the dividing-line, it seems desirable to 

 take the lower denomination as the term. 



A proportional triangle, such as that given by Pierre Keynes in 

 the forefront of his Monograph on Ammonites (1879), is suitable 

 for taking the measurements. 



EXPLANATION OF PLATES XXVII & XXVIII. 



[The specimens are drawn of the natural size.] 



Plate XXVII. 



Variabilis heniera. 

 Denckmannia bredonensis, S. Buckman, sp. nov. 

 Fig. 1. Side view. 



2. Peripheral view. 



3. Suture-lines. 



Figs. 4 a & 46. Kadial curves. 

 The specimen is from the gravel-pit at Overbury (Worcestershire), and is in 

 the collection of Surgeon-Major Isaac Newton. 



Armati hemera. 

 Deroceras sp. 

 Fig. 5. Side view, showing the bituberculate stage, with the outer row 

 of spines gaining at the expense of the inner. See Biological Note, 

 p. 460. 



6. Peripheral view. 

 The specimen is from the Lias of Lyme Eegis [Pliensbachian], and is in my 

 collection. It illustrates the phylogeny of Deroceras armatum (Sow.). 



Plate XXVIII. 



Dispansi hemera. 

 Chartronia costigera, S. Buckman, sp. nov. 

 Fig. 1 . Side view. 



2. Peripheral view. 

 Figs. 3 a & 3 b. Suture-lines. 

 Fig. 4. Eadial curve. 

 This specimen is from the Dispansim-Bed of the Cotteswold Cephalopod-Bed 

 (Toarcian) of Buckholt Wood, near Stroud (Gloucestershire), and is in the 

 collection of Mr. Charles Upton. 



Discussion (on the two foregoing papers). 



The Chairman (Mr. E. T. Newton) remarked that he could say little 

 as to the Bredon-Hill section, except that a map executed 50 years 

 or so ago naturally required considerable modification. 



Mr. Httdleston, referring exclusively to the first paper, had very 



