part 4] THE EVOLUTION OF THE LIPAROCERATID.E. 



267 



The protoconch shows the usual characters (PI. XXI, fig. 3 d)> 

 its length being 0*68 mm. Shell development proceeds normally 

 for some time ; thus there is a general increase in the size of the 

 whorl, whorl height at first increasing proportionately more rapidly 

 than whorl thickness. The shell remains smooth until a diameter 

 of o mm. is reached, when there appear on the sides widely- 

 separated ribs, which do not cross the venter until later (diameter 

 10 mm.). The whorl thickness increases regularly, but the whorl 

 remains depressed much longer than is usual in capricorns (see 

 PL XXI, tig. 3 a) which generally acquire slender, nearly round 

 whorls at a diameter of 3 mm. Thus, it will be seen from the 

 following table that, in Liparoceras sparsicosta, the whorl height is 

 less than the whorl thickness, even at a diameter of 20 mm. This 

 retention of the early depressed shape and smooth whorls may point 

 to the primitive nature of L. sparsicosta ; it is probable, however, 

 that these features indicate that this ammonite is already beyond 

 the true Capricorn stage, and that the depressed inner whorls are 

 due to the prolongation of the primitive globose form. 



Diameter. 



Whorl height. 



Wh 



orl thickness. 



Umbilicus, 



' mm. 



per cent. 





per cent. 



per cent. 



•58 



48 





119 



— 



•68 



42 





96 



28 



114 



42 





69 



30 



1-8 



40 





65 



— 



3-6 



39 





55 



29 



8-0 



28 





55 



— 



160 



28 





46 



36 



20-0 



33 





45 



35 



The first suture shows typical angustisellate characters (text-fig. 

 2a). At the second suture the azygous saddle is divided by a fairly 

 deep ventral lobe, which is deeper than the corresponding lobe in the 

 second suture of other capricorns (text-fig. 2b). In the third suture 

 the ventral lobe has a small median saddle ; there is little advance 

 on this stage during the next whorl, the median ventral saddle 

 becoming slightly more pronounced and an auxiliary saddle appear- 

 ing at the umbilical margin (text-fig. 2 d). A very feeble denti- 

 culation appears on the dorsal margin of the external saddle at a 

 diameter of 4*5 mm., and a little later the saddle has three terminal 

 cells and the first lateral lobe three lobules. Subsequent develop- 

 ment leads to the increase in height of the external saddle and in 

 depth and width of the first lateral lobe ; these attain adult pro- 

 portions at a diameter of 10 mm., all the elements later becoming 

 complicated by minor denticulations. 



Distinctions. — Androgynoceras maculatum is the only Capri- 

 corn among the British Liparoceratidse from which Liparoceras 

 sparsieosta is not readily distinguished : A. maculatum has similarly 

 widely-spaced ribs (about seventeen to a whorl ; some capricorns 

 may have thirty) which are feeble on the venter. In A. 

 maculatum, however, whorl height is greater than whorl thickn 



