446 INFERIOR OOLITE AMMONITES. 



similarity of external form — is a most misleading thing, yet it is sometimes so 

 striking that unless care be exercised it will be certain to mislead, as it has misled 

 nearly every beginner in the making of classifications, especially in the case of 

 Ammonites. 



It should be noted that it is the object of the present generic classification to 

 express our ideas about genealogy. These ideas are theoretical, but only in the 

 same way as the statement that a man with a certain type of nose is of Jewish or 

 Negro descent. In the matter of, say, poultry, a breeder can tell the descent from 

 the characters of the hen before him ; and the Ammonite specialist claims no more. 

 It must, however, be borne in mind that it is not the characters which determine 

 the descent, but the descent which has determined the characters. A and B were 

 parents of C, and D and E were not. We do not know whether A, B or D, E were 

 the parents, but we try to interpret the characters of C so as to find out. It may 

 have been stated that the descendants of A, B, all had a certain character, and we 

 may be expected to class C with D, E because it does not show this character as 

 it ought to ; yet the absence of the character can never make C the offspring of 

 D and E if it was really the offspring of A and B. It is the definition which is at 

 fault. Other characters may show plainly that C must be classed with A, B, and 

 can never have belonged to stock D, E, just as a very slight feature in an animal 

 betrays to the experienced breeder a fact in the beast's ancestry, no matter how 

 strongly other characters may seem to point to the contrary. 



Setting aside its genealogical characters, Table IX is constructed to show at 

 a glance the affinities of the various species, and, as they are placed in vertical 

 position to correspond, as far as possible, with their different degrees of retro- 

 gression from the spinous, gibbous-whorled, evolute form, it indicates at once 

 those species which possess nearly the same external similarity of form. Practi- 

 cally speaking, all the species which are in the same horizontal line are externally 

 homceomorphous, while all those which are in the same vertical line are internally 

 homceomorphous, and possess, besides, such similarity in general as may be 

 supposed to accompany a definite direction in development. 



It might be supposed that because all these species are described as " from the 

 Concavum-zone" therefore they were contemporaneous. Such an assumption is 

 hardly admissible ; it is based upon the supposition that the strata of what has 

 been called a zone, or any slight thickness of Ammonitiferous rock, had been 

 deposited during a very short space of time. 1 This cannot have been the case — 



1 In the ' Great Oolite Mollusca,' p. 3, Palseontographical Society, 1850, Morris and Lycett 

 remark, " The beds of closely-packed Ammonites, of every stage of growth, which occur in certain of 

 the Jurassic rocks, would appear to be the effect of occasional rapid earthy deposits which took place 

 during that seasonal period when the molluscs lying torpid and contracted within their shells were 

 at once entombed in that condition." The statement is repeated in the 2nd edit. ' Geology of 

 England and Wales,' by H. B. Woodward, 1887. Yet when these thin beds have been traced 



