134 PALEONTOLOGY 



backward curves are lobes. The visible suture-line (without 

 the portion on the impressed area) consists of (i) a 

 median peripheral or external lobe, divided into two by a 

 small pointed saddle, (2) a pair of external saddles on the 

 peripheral margins, followed on each side by (3) the first 

 or superior lateral lobe, (4) the first or superior lateral saddle, 

 (5) the second or inferior lateral lobe, (6) the second or inferior 

 lateral saddle, (7) an auxiliary lobe, (8) an auxiliary saddle, 

 (9) another auxiliary lobe. Both lobes and saddles are 

 much subdivided, but the numerous subdivisions are all 

 approximately equal. The first lateral saddle is much 

 larger than the second, and lies but little above the 

 centre of the lateral area. 



If we draw a straight line from the centre of the 

 ammonite to the ends of the external lobe (normal line or 

 guide-line), we see that all the other lobes fall short of this 

 line, the first lateral lobe most of all. 



4. Xipheroceras planicosta is another ammonite 

 from the zone of A. obtusum. It is common at Lyme 

 Regis, but far more beautiful specimens are, or were, 

 found in a bed of limestone at Marston Magna near 

 Yeovil the " Ammonite Marble " or Marston Marble, 

 at one time much used for ornamental purposes, and still 

 generally obtainable in small fragments from mineral 

 dealers. In both localities the usual specimens are 

 small (diameter about 2 cm.), larger ones (5 or 6 cm.) 

 are rare and probably not identical in species. The 

 dimensions of one of the small size are 



20 : 33, 32-36, 45. 



