

342 



ME. L. F. SPATH OX THE 



[Dec. 1 9 14, 



Fig. 3 a, PL XLIX, represents a high-zonal form (35 feet above 

 the Belemnite- Stone : that is, horizon of the ' Upper Limestone ' 

 on Stonebarrow Cliff) ; but, except in the suture, it differs in no 

 respect from the ammonites occurring below. It will be noticed 

 that the whorl-section reaches the circular stage at a diameter of 

 2 mm., just as it does in Psiloceras plcmorhis ; whereas in the 

 Triassic Pompechjites layeri (Hauer) it is reached even earlier. 

 Lunuloceras lunula (Zieten), on the other hand, does not become 

 circular in section until a diameter of 4 mm., indicating a long 

 series of evolute, depressed ancestors. These figures also illustrate 

 the transition from, the rounded umbilical edge of the widely- 

 umbilicated young shell to the perpendicular wall of the larger 

 specimens, which at a diameter of 6*25 mm. already (fig. 4, 

 PL XLIX) resemble in this respect the adult type (fig. 1, 

 PI. XLIX). 



The siphuncle, which is exceedingly wide, pierces the septa first 

 ventrally (fig. 2 c, PL XLYIII), then subcentrally (fig. 6, PL 

 XLVIII), and does not resume its ventral position until near the 

 tenth chamber. The distance between the septa is very constant ; 

 there are fourteen septa to the whorl at a diameter of 09 mm. 

 (fig. 2 a, PL XLIX, to second constriction), fourteen at 2*8 mm. 

 (fig. 3 a, PL XLIX), and fourteen again at 6*25 mm. (fig. 4 c, 

 PL XLIX). 



Table I. (Change of the shape of the whorl coincident 

 with the increase in diameter.) 





Diameter 

 in millimetres. 



Height. 

 Per cent. 



Thickness. 

 Per cent. 



Umbilicus. 

 Per cent. 



No. 1 (fig. 7, PI. L) 



2 (fig. 8) 



2a(fig. 9) 



3 



1-2 



2-8 



6-25 



5 



6-5 



7 



7-5 



8 



8 

 11 

 11 

 11-5 

 12 

 13 

 13-4 

 14-25 

 15 

 17 

 18 

 19 

 20 

 25 

 38-5 



35 

 43 

 55 

 55 

 54 

 54 

 54 

 53 

 53 

 52 

 52 

 51 

 50 

 54 

 49 

 54 

 53 

 50 

 51 

 52 

 50 

 53 

 53 



44 

 40 

 35 

 37 

 31 

 30 

 29 

 28 

 31 

 27 

 27 

 26 

 27 

 31 

 30 

 27 

 26 

 24 

 25 

 23 

 23 

 26 

 24 



37 

 33 

 20 

 22 

 22 

 21 

 19 

 22 

 20 

 20 

 16 

 17 

 16 

 15 

 21 

 16 

 15 

 21 

 16 

 16 

 16 

 14 

 13 



4 



5 



6 • 



7 



8 



9 



10 



11 



12 



13 



14 



15 



16 



17 



18 



19 



20 



21 



22 





