142 Proceedings of the Royal Society of Victoria. 



DESCRIPTION OF THE SPECIMEN. 

 Desinoceras, Zittel. 

 Desmoceras jonesi, n. sp. 



Diagfwsis. — A large and discoid Desmoceras, with flat sides; 

 young whorls marked by broad, rounded, well developed ribs. 

 Back narrow, bluntly rounded. Umbilicus broad, the steps over 

 the successive whorls are steep ; about f th of each whorl is 

 exposed below the next outer whorl. Mouth long, with the 

 sides in part subparallel. 



Sutures : on the outer whorl there are four paired lateral 

 saddles. The siphonal saddle is short and broad, it has 

 one projection on each side ; and it appears bicornate owing 

 to a lateral projection at the upper corners The first lateral 

 saddle is very large and complex. It consists of four main 

 divisions, of which the two middle ones are the largest and most 

 complex ; they both arise as branches from a narrow stem. The 

 base of this saddle is large, it gives off" the first branch on the 

 dorsal side. This branch is simple but deeply notched. The 

 second branch is on the ventral side, and is divided nearly to the 

 base by a notch which divides it into a simple and a compound 

 sub-branch. The last division begins with a narrow stem, and 

 this dichotomizes to irregular and very sinuous sub-branches. 

 The second lateral saddle is nmch smaller ; it has two main 

 simple branches on the ventral side and a compound division on 

 the dorsal side. The third lateral saddle is slightly smaller than 

 the second ; it is divided into two divisions. The fourth lateral 

 saddle is small and consists of three simple divisions.^ 



The notches on the lobes are comparatively broad and have 

 blunt rounded ends. 



Dimensions. 



Largest diameter of specimen - 25 cm. 



Proportion (wliole diam. =1.) 



Outer vvliorl : whole diam. - = .48 



Width of uml>ilicus : whole diam. = .28 



1 The saddles within the line of involution are not shown in the specimen. 



