454 THE LIAS AMMONITES. 



iiess as they advance, and terminating abruptly by the side of the carina ; the size and 

 regularity of the tubercles, the straightness of the ribs, and the manner they end by 

 the side of the thick round keel, distinguish this species from all other congeneric forms. 

 When it attains a diameter of 120 millimetres the tubercles begin to disappear, the costse 

 fine down into depressed radii, and the keel becomes blunt and disappears, the outer 

 whorl and the body-chamber change their form and become triangular, and the shell 

 assumes a form which I have represented in PI. LXVI. Just about the point where this 

 metamorphosis takes place we notice on the mould a remarkable constriction, always in 

 that portion of the shell where the septa are present ; it is always unique, and never 

 present before or after. At one time I was of opinion that the constriction might have 

 been the result of an accident to the moUusk or to its shell during life ; afterwards, how- 

 ever, I observed like constrictions in other specimens at the diameter of 120 millimetres, 

 and hence it occurred to me that the Ammonite changed its lines of growth when it had 

 attained that magnitude, just as we have a similar metamorphosis at a definite age in Aego- 

 ceras heterogenum of the Middle Lias. The two specimens delineated in PL LXV, figs. 1 

 and 4, show the constriction described, and a large imperfect specimen, 210 millimetres 

 in diameter, from Heiningen, Wlirtemberg, in my collection, has a little mark on the 

 mould at 120 millimetres diameter. In both the figured specimens referred to, the form 

 of the whorl changes beyond the constriction ; the whorls which up to that mark had 

 been of a rounded or eUiptical figure, begin henceforth to assume a distinctly triangular 

 outline. In the beautiful and accurate plate of Am. insignis given by d'Orbigny (pi. 112) 

 this fact is very well represented, although nothing is stated in the text relating to the 

 change of form by that most accurate observer, who appears to have had a large series of 

 specimens illustrative of the morphology of Harp, insigne. On this subject our author 

 states that " this species is without contradiction one of the most remarkable in reference 

 to its changes of form according to the age and sex of individuals. An example of 

 the diameter of 4 millimetres is entirely smooth, with a round back without keel. 

 Another of a diameter of 5 millimetres presents some slight prominences around the 

 umbilicus, and preserves them for some time without an indication of a carina. When 

 the diameter has reached 10 millimetres the lateral costse, the tubercles, and the carina 

 become conspicuous. At the diameter of from 15 to 20 millimetres fourteen tubercles 

 and thirty-two costae can be counted. There are seventeen tubercles and thirty-eight 

 costse when the shell is of the diameter of 50 millimetres. A compressed specimen at 

 the diameter of 125 millimetres showed thirty-four tubercles and eighty-four costse. 

 The period of degeneracy commences sooner or later according to individuals. With a 

 diameter of 180 millimetres in bulky inflated specimens the tubercles sometimes cease ; 

 the ribs at 210 millimetres ; the remaining portion being very smooth throughout." 



I am able to supplement this account of the morphology of Harpoceras insigne by 

 facts disclosed during the study of an interesting example of this Ammonite collected 

 from the light-coloured marly beds of Upper Lias at Sheepscombe, near Painswick, 



