92 



Report of the State Geologist. 



Figures 70, 71. Probeloceras Lutheri. 

 Front and lateral views of the proto- 

 conch and first whorl, x 25. 



The Conch. With the commencement of the first volution, the whorl 

 section is transversely semielliptical and about twice as wide as high. As 

 observed already, the size of the whorl section diminishes from its inception, 

 as in Mantic. Patter soni, but the decrease and subsequent increase are not so 

 marked as in that species. Taking corresponding whorls of these two species, 

 we shall find that at no time is the whorl section of the former relatively as 

 broad as in the latter. In Probel, Lutheri, increase vertically goes on apace 

 and the section at the end of the second volution is fully as long (dorso- 



ventrally), as the section at the end of the third 

 volution in Mantic. Patterson /. So far as the final 

 form of the whorl section in Mantic. Patterson i is 

 reproduced in this species, it is found in the course 

 of the third volution, but even here has superinduced 

 upon it the ventral flattening which especially 

 characterizes all subsequent growth stages and, in 

 Mantle Pattersoni, attains in eventual conditions 

 somewhat the same degree of prominence as here 

 in this early stage of Prohel. Lutheri. We shall presently notice more 

 at length, the variations accompanying the development of the ventral ridge 

 in this species ; its effect on the form of the whorl is most sharply marked in 



the fourth and fifth volutions, and there- 

 after it becomes a somewhat less defined 

 feature of the conch. 



I r mbilication. Notwithstanding the 

 large umbilication of this species, we 

 find here even more forcibly exemplified 

 than in other species here discussed, 

 the regular increase in the amount of 

 overlap in successive whorls. This is 

 continued to a definite growth-stage 

 when it is followed by a rapid in- 

 crease in umbilication to the final stages. 

 In the figure 77, on page which 

 shows in section all the whorls in a 

 normal adult, this variation in um- 

 bilication or degree of overlap is evi- 

 dent and may be expressed as follows, beginning with the overlap of the 

 1st by the 2nd whorl; .30, .33, .40, .41, .24. Thus there is an increase up 



Figure 72. Frobeloceras Lutheri. This is a composite 

 figure consisting of seven outlines of the early volutions 

 similarly oriented with reference to the protoconch 

 and primary curvature. It shows the variations in 

 degree of umbilication existing within the species. 



