54 TYPE AMMONITES— IV Aug. 



may be the subsequent total removal of beds over large areas without 

 leaving any trace is seen in the remarkably small patches of strata of 

 niortensis hemera which are preserved : its sea must have made deposits 

 over quite a wide expanse, little less than the area shown as water for 

 the Stepheoceratan Age. In many cases, the length of time for such 

 removal without trace was much less than a hemera ; but in regard 

 to the removal of Jurassic strata from Palaeozoic rocks there can often 

 have been the time of Epochs, Periods, or much longer. 



Systematic 



To illustrate the method of working with regard to generic names 

 there may be given here a series of short diagnoses. 



Family MACROCEPHALITIDM 



1. Catacephalites, pi. CCLXXXIII. Sub-sphaeroconic ; flexicostate, 



ribs rather coarse ; suture-line sub-simple, lobes short, EL = Li. 



2. Kamptokephalites, pi. CCCXLVII. Compressed ; flexicostate, 



ribs coarse ; s. 1. feeble, EL longer than Li. 



3. Macrocephaliceras, pi. CCCXIII. Sphaeroconic persistent into 



smooth stage ; flexicostate, ribs medium size ; s. 1. highly 

 developed, EL=Li. 



4. Pleurocephalites, pi. CCLXXXIV. Sphaero- to subsphseroconic ; 



flexicostate, ribs fairly strong ; s. 1. well developed, with long 

 lobes, EL =Li. 



5. Dolikephalites, pi. CCCLXXIL Compressed ; flexicostate, ribs 



small, numerous ; s. 1. feeble, EL longer than Li. 



6. Macrocephalites, PI. CCCXXXIV. Sub-sphaeroconic ; recti- 



costate, ribs small, numerous ; s. 1., lobes short, but fairly 

 ornate, EL=Li. 



7. Tmetokephalites, PI. CCCLXXIIL Compressed ; flexicostate, 



ribs small, numerous; s. 1. very elaborate, EL=Li. 



The differences between these genera should be fairly obvious ; 

 but they may more readily be grasped if put into tabular form, taking 

 just the three characters, shape, degree of costation and complexity 

 of suture-line, giving to each a numerical value greater for its departure 

 from a supposed common primitive form. Thus the coarser the costation, 

 the lower the number ; the more complicated the suture-line, the higher 

 the number. 



Table A — Macrocephalitid Genera 



The totals, therefore, give the natural order. 



These genera occupy different deposits — Cornbrash, genera 2 and 5, 



