254 THE EVOLUTION OF THE LIPAEOCERATIDJE. [vol. lxxiv. 



IV. GrENEBAl EVOLUTION. 



It is shown in the preceding notes that in the upper zones of 

 the Lower Lias occur ammonites of several genera, which evolve 

 from slender capricorns with stout ribs to involute bituberculate 

 forms with swollen globose whorls, and that the morphic equiv- 

 alents on separate lines of descent are so similar in certain cases 

 as to present some difficulty in identification, unless sutural and 

 other minute characters are taken into consideration. Further, 

 the several series are not quite contemporaneous, morphic equiva- 

 lents of different genera occurring at considerable intervals, and 

 thus the precise geological age of one of these ammonites can only 

 be determined when its lineage is known. It will be convenient 

 now to consider the general trend of evolution in the characters 

 which are common to all the series. 



Early capricorns of this family show lateral ribbing at a 

 diameter of 3 mm. or thereabouts, no ribs being present on the 

 periphery until much later. In more advanced capricorns, how- 

 ever, the ribs appear, by tachygenesis, at about the diameter of 

 2 mm. 



In the latter forms, at a diameter of about 100 mm., the ribs of 

 the last whorl are frequently more widely-spaced, and may acquire 

 small tubercles. In slightly later members of each series this 

 condition is accompanied by a rapid expansion of the last whorl, 

 the ribs becoming more slender and bearing paired tubercles ; the 

 ammonite is then of the hetei*ogejies-type. The whorls are at 

 first smooth and thick, become slender and only slightly embracing, 

 later becoming stout again, the development of this, latter condi- 

 tion thus being indirect. From this point onwards the tendency 

 in evolution is to shorten the period of slender costate whorls and 

 to huriy on to the bituberculate condition. This is accomplished by 



(1) The acceleration of the development of the (secondary) swollen 



whorls, and of the bitubercnlation. 



(2) The prolongation of the stage of smooth and globose whorls. 



As a result, the stage of slender whorls with Capricorn ornament 

 is ultimately ' skipped ' entirely in development. An example 

 will make this clear. In specimen A, shown diagrammatically in 

 Table II, Capricorn ornamentation is developed at a diameter of 

 2 mm., and the shell does not become bituberculate until a diameter 

 of 90 mm. Succeeding forms (B, C) retain the smooth globose 

 whorls until diameters of 3 mm. and 5 mm. respectively, the bi- 

 tuberculation commencing earlier in each ; thus the length of 

 whorl with Capricorn ornament in successive specimens of a lineage 

 gradually becomes shorter, until ultimately bituberculation appears 

 without any previous Capricorn stage (D, E). In the last- 

 mentioned forms the primitive smooth condition is retained much 

 longer than in the immediate ancestors. 



The occurrence of such a series of specimens on each of the 

 lineages examined confirms the conclusion that the smooth inner 



