189C,] ^^i ^ [Smith. 



representative of the genus Gastnoceras. But there are species of Oas- 

 trioceras tliat are globose and not flattened, and without the umbilical 

 ribs or nodes ; also certain species have their sutures very angular. On 

 the other hand certain species of Glyphioceras have weak umbilical 

 nodes and rounded sutures. 



This species is so closely related to Oastrioceras lister i Martin, sp., 

 Petrif. Derb., PI. xxxv. Fig. 3, that they have been united by Gurow. 

 Others still are inclined to unite it with Glyphioceras diadema Goldfuss, 

 while many would join it with Oastrioceras jossm Verneul. 



From G. josscB it differs in the almost total absence of spiral ribs or 

 striae, and in the wider and more angular umbilicus, but they are so 

 similar that G. marianum may be considered the ancestor of G. jossce. 



The best mark of sejiaration from G. listeri is the greater number of 

 coils which G. marianum has, as many as seven being known on a 

 specimen of less than one inch in diameter. 



G. kingii Hall and Whitfield, U. 8. Expl. Fortieth Parallel iv, p. 279, 

 PL vi, Fig. 9-14, is a closely related form, but diff'ers in having the 

 umbilical slope a little more gentle, the angle with the axis of the shell 

 being 40-45°, while that of G. marianum is about 37°. G. kingii has 

 fewer whorls to the same diameter. G. marianum also has the external 

 saddle not so deeply divided, and the two siphonal lobes are wider and 

 become somewhat broadened at the ends. The ribs on the sides of 

 G. marianum are much stronger. Weak spiral strife are seen on the 

 inner whorls. 



The transverse lines of growth form incipient undulations on the 

 ventral portion of the shell. Strong constrictions occur both on the 

 cast and on the shell, on the body chamber, as well as on the rest of 

 the chambers, becoming weaker with age ; their number is about three 

 to a whorl, and the^^ curve forward, with a gentle sinus pointing back- 

 ward. 



The ribs are strong on the sides, forming sharp nodes or tubercles, 

 and are continued across the ventral portion by fine undulations. 

 Towards the centre or umbilicus the ribs weaken very suddenly. The 

 sutures are like those figured by Verneul, but show also the small 

 "suspensive" lobe on the umbilical border, as described and figured 

 by Karpinsky. 



The body chamber is at least one coil in length. 



Dimensions. — Some fragments indicate a size of not less than two and 

 a half inches in diameter. The most perfect specimen has the follow- 

 ing dimensions : 



MM. 



Height of last whorl 9 



Diameter , 30 



Width of umbilicus 14 



The breadth of the last whorl is about two-thirds of the diameter of 



