A. H. Noble — An Ammonite from Chalk Rod;. 



399 



and shallow, its diameter (measured from the spiral suture) being 

 rather more than one-third of the diameter of the shell. The 

 peripheral area is broadly convex, sloping more steeply down to the 

 point of maximum breadth of the whorl in cross-section, which is 

 situated at about two-fifths of the radial diameter measured from the 

 umbilical margin. The umbilical zone is fairly large, sloping steeply 

 down to the spiral suture. 



There is a series of constrictions, numbering about eight on a whorl. 

 The course of these is straight up to the siphuncle, where the ligation 

 becomes directed very slightly forward. No ribbing is visible on the 

 cast, but there are some traces of nodular swellings, each one situated 

 posteriorly to a constriction at the outer limit of the umbilical slope. 





Fig. 1. Desmoceras marlowense, sp. nov. a, front view; b, lateral aspect. 

 Chalk Rock : Marlow, Buckinghamshire. Both nat. size. 



The suture-line is well seen on several parts of the specimen, and 

 can be traced from the siphon al line as far as the second lateral lobe. 

 The external saddle is divided into two parts by a fairly deep lobe, that 

 part nearer the siphuncle being slightly greater than the other part. 



The first lateral saddle is about as high as the external saddle, but 

 is simpler. The first lateral lobe is trifid and of the same length as 

 the ventral or siphonal lobe. The second lateral lobe is similar to the 

 first, but shorter and simpler. 



Dimensions. mm. 



Greatest diameter . . . . . 51 



Height of outer whorl ..... 21 



Thickness of outer whorl in cross-section . 25 



Width of umbilicus . . . . . 19 



Locality and Horizon. — The specimen was found in a small pit 

 about one-quarter of a mile E.N.E. from Blount's Farm, Great 

 Marlow. Here the whole of the Chalk Rock is exposed, and a very 

 rich fauna was obtained from a band which occurs just above the green 

 nodular band about 2 feet from the top of the Chalk Rock zone of 

 Holaster planus. 



