328 E. W. Berry — An American Spirulirostra. 



The guard or rostrum terminates behind as a smooth 

 conical point. As it expands forward the rounded ventral 

 face develops into a prominent boss, above which it is 

 excavated below, and widened on either side into wide 

 thick flanges, which form a sweeping forward curve to 

 where they unite with the proostracum in front of the 

 phragmocone. The boss of the rostrum and the margins 

 of its wings are prominently mammilated, the surface 

 reflecting the vascular structure of the enclosing mantle 

 of the adult animal. 



On the dorsal side the rostrum commences to flatten at 

 a point about opposite its ventral boss, resulting in 

 diverging rounded shoulders, flat top, and flattened 

 sloping sides. The surface of the top becomes smooth 

 as it is continued forward but the sides show subordinate 

 vascular markings. The forward continuation of the 

 rostrum forms a thick wall about the phragmocone, the 

 earlier chambers of which form a subordinate boss, cen- 

 trally located in the ventral concavity of the rostrum, 

 some distance in front of the prominent ventral boss. 

 The former is usually broken in the fossils, and the earlier 

 chambers of the camerated shell can be seen through the 

 resulting opening. 



The protoconch is bulbous like those of the ammonites, 

 and is not embedded in the ventral boss of the rostrum 

 as it is in the other known species of Spirulirostra. The 

 conch for the first five or six chambers is tightly coiled, 

 endogastrically, but there is no impressed zone. The 

 diameter of the chambers increases very rapidly as they 

 curve upward away from the nepionic coiled portion, and 

 then forward, as shown in the accompanying diagram- 

 matic longitudinal section. In all there appear to have 

 been 15 or 16 septa, which were probably transverse and 

 nearly straight in profile, although they are broken away 

 except near the sutures in every specimen seen. The 

 siphon was small and followed the ventral wall. The 

 phragmocone is circular throughout in cross section. 



The proostracum is a thick, spatulate forward projec- 

 tion from the dorsal part of the rostrum, nearly flat 

 above, with rounded edges, and mammilated surface, 

 which becomes more conspicuously so toward its anterior 

 rounded margin. Viewed from below it shows two flat 

 lateral wings, each occupying about one fourth of its 



