AMALTHEUS MARGARITATUS. 399 



points for digitations. A radial line extended from the umbilicus to the siphonal lobe 

 would pass through the middle of the principal lateral and touch the points of all the 

 others. 



The evolution of this Ammonite is extremely interesting. Several years ago I obtained 

 a series of young shells from the Amal. marffaritatus-hG& near Reutlingen, Wiirtemberg, 

 which show the morphological phases through which the shell passes. When it has attained 

 the diameter of 10 millimetres, the sides of the whorls are round, and the straight ribs 

 develop small tubercles about the middle, each rib then curves forward towards the keel 

 (PI. LVI, figs. 13 and 14) ; in the next stage (figs. 11, 12) the ribs are better defined, and 

 the tubercles more prominent ; the siphonal area is proportionally much wider and forms 

 a ridge with two angles as is shown in the aperture (fig. 12). Another variety, about the 

 same age, is delineated in figs. 8, 9, the whorls are wider, and the ribs and tubercles more 

 pronounced in figs. 19, 20. When the diameter of 17 millimetres is attained the whorls are 

 still roimd, the ribs distinct, and develop a tubercle near the middle of their height, the 

 ribs then bend sharply forward towards the aperture ; the keel is now prominent with 

 ridge-like, short, supplementary ribs set obliquely forward between the primaries. Fig. 

 10, with a diameter of 22 millimetres, has the ribs well-marked, the tubercles blunted, 

 and the outer half of the whorl encircled by fine longitudinal lines ; the keel well marked 

 and carinated, is notched by the angular ridge-like pieces. Figs. 6, 8, representing a 

 form of 20 millimetres in diameter, has all the parts of the shell already described more 

 fully developed, and the siphonal area exhibits its sharp prominent ridge and angular 

 sides, bounded by a prominent tubercle on each side. A specimen before me, 31 milli- 

 metres in diameter, is a perfect example of the form figured by Quenstedt as Amaltheus 

 coronatus, which has round whorls, short thick ribs, prominent tubercles, and two 

 branches from each inclined towards the aperture ; the keel prominent and nodulated, is 

 altogether a very pronounced variety of the usual type. When the shell has attained 40 

 millimetres in diameter it then becomes Amal. depressus, Quenst. The ribs are biflexed 

 and flat ; the keel rope-like and carinated, and the shell ornate with longitudinal punc- 

 tated lines which encircle the outer whorl ; the spines begin now to disappear, though 

 they are seen all round the turns of the inner whorls when we examine the umbilicus. 

 When the tubercles cease to be developed the shell assumes the form of its adult condition. 

 In another specimen, of 70 millimetres diameter, the shell has attained all the characters 

 so well displayed in the large adult shell, beautifully delineated in PI. LIII. 



Affinities and Differences. — The remarkable form which the keel assumes in this 

 species resembles Amaltheus ibex, but is distinguished by its narrower area, acute rope- 

 like carina, more triangular whorls, and more complicated lobe-line ; some of its varieties 

 assume shapes that resemble Amal. sj)inatus, but the ribs and spines of that species, with 

 the thickness of its whorls, and the breadth of the siphonal area, form distinguishing 

 characters by which the two species may be readily diagnosed from each other. 



Locality and Strati^raphical Position. — I have collected fine specimens of this 



