94 



Report of the State Geologist. 



front and behind, the lamella? appear as a pair of crests bordering the lateral 

 ridges of the ventral area, though they at once resolve themselves into the 

 structure shown in such a view as figure 3, the lines bending retrally over 

 the median ventral groove. It is probable that these lines become conspicu- 

 ously elevated at a somewhat earlier stage than that represented in figure 2, 

 but they are resorbed in consequence of overlap, and it will be observed that 

 for a half-volution in this specimen these lamellae have escaped resorption. 

 In other specimens the ornamentation over the fifth volution consists of the 

 primary stria? with intercalary stria? of somewhat irregular order and number, 



\ 



Figure 75. Probeloceras Lutheri. The inner whorls, drawn from an 

 impression of the exterior. This figure shows the protracted nepionic 

 stage with its terminal varix and the mode of introduction of varices 

 on the neanic whorls, x 25. 



the entire striation becoming more and more faint outward. Soon it disap- 

 pears or is resolved into a uniform striation of the surface, which in the final 

 condition may assume the aspect of an obscure undulation or pilation of the 

 surface. The ventral lines remain sharp and strong throughout, though with 

 gradual loss of intensity. 



The hyporwmic flait&n/mg. On account of the rapid change of form during 

 the course of the first and second volutions, the whorl soon (two and one-half 

 volutions) attains the outline characteristic of mature Mantic. Pattersoni. Even 

 before this, and as early as the beginning of the second volution the ventral 

 flattening is well defined, as shown in figure 73, where it is seen to be a groove 

 bounded by elevated margins. Where the surface of the previous whorl has 

 been overlapped and the lamella? resorbed, the definition of the hypnomic 

 flattening is obscure though apparent. 



During the course of the fifth and sixth whorls the depth of the median 

 groove and the elevation of its lateral ridges greatly increase, but thereafter 



