830 THE LIAS AMMONITES. 



This regularity of succession, which the Semur Collection appears to prove, accords very 

 well with what has been observed in other groups. Not only does the involution greatly 

 increase in each succeeding species, but the septa become more complicated in outline, 

 and the adult characteristics of the ribs and form are repeated at earlier and earlier 

 stages in each species ; the same law governs also the inheritance of the old-age charac- 

 teristics of the individual. Thus, Aeg. BoucauUianum, which occurs latest in time^ 

 has the old-age characteristics sooner developed in its growth than any other form, 

 showing that the acceleration or quicker reproduction of the characteristics extends to 

 the whole life, affecting even the period at which old age begins. 



Aegoceras LA.CUNATUM, Buckman. PI. LVI, figs. 16, 17, 18. 



Ammonites LACUNATTJS, BmcAmow. Geology of Ckeltenham, 2ncl ed., p. 105, pi. 11, 

 figs. 4, 5, 1845. 



— — Quensfedt. Cephalopoden, vol.i, p. 151, pi. xi, fig. 13, 1849. 



— — Quenstedt. Der Jura, p. 98, pi. xii, figs. 4 — 6, 1858. 

 Ammonites iacitnatus, Dumortier. Depots Jurass. Bassin du Rhone, vol. ii, p. 120 



pi. xxi, figs. 18—20, 1867. 



Diagnosis. — Shell small, compressed, not carinated, composed of highly involute 

 whorls, which are convex on the sides, of a uniform thickness throughout, and 

 ornamented near the umbilicus with twenty-five to thirty short ribs, separated by deep 

 valleys ; the primary ribs soon divide into two or three branches ; this division does not 

 take place on all the ribs either regularly or at an equal distance from the umbilicus ; 

 often a second bifurcation takes place ; the ribs are then strongly inflected towards the 

 aperture, and form graceful curves on the sides of the shell, ending in from sixty to 

 seventy costse at the outer margin of a deep sulcus which occupies the middle of the 

 siphonal area, ' and interrupts the passage of the costse of the right towards that of 

 the left side of the area; umbilicus very narrow. 



Dimensions. — Diameter 30 millimetres; height of the last whorl 16 millimetres; 

 width of the aperture 8 millimetres ; amount of involution two thirds of the penultimate 

 whorl. 



Description. — This rare little Ammonite was first discovered by Professor Buckman 

 near Cheltenham. It has been thought by some to be the brood of Jeg. Charmassei, 

 and by others to be a young Aeg. BoucauUianum. It differs, however, specifically from 

 both, and is an interesting form of the section Angulati. 



The shell is small, compressed, and not carinated ; the whorls, four or five in number, 

 are inflated, convex on the sides, extremely involute, and embrace two thirds of a whorl 



