Clarke — The Naples Fauna. 



95 



it becomes n less prominent feature, gradually losing its concavity and the 

 sharpness of its lateral elevations, until, in final stages it is broadly convex 

 medially with narrow depressions on the outer surface between the median 

 convexity and the sides. 



Septation. We have not been successful in isolating the protoconch of 

 this species in such a condition as to elucidate the complete form of the first 

 septum. The barite replacements seldom show the early septa and the same 

 is true of the pyrite cast. Some of our figures show the direction of the first 

 septum over the exposed surface of the whorl, but its course upon the venter 

 has yet t<> be determined. During the course of the first volution only the' 

 very broad curve of the lateral lobe is exposed. At the end of this volution, 

 at the seventh septum, the course of the suture is as follows : There is a broad 

 ventral lobe dividing two low and obtuse ventro-lateral saddles; these are 

 followed below by long, low, retral curves, forming the lateral lobes. The 

 dorsal course of the suture is simple, the dorsal 

 lobe being the only interruption of its simplicity. 

 At one and two-thirds volutions and up to two 

 and one-third volutions there is little alteration 

 in the form of the suture, but directly there- 

 after the broad ventral lobe is divided by the 

 introduction of a ventro-lateral saddle, which 

 necessitates the appearance of a corresponding 

 ventro-lateral lobe ; these are never of equal 

 size, the former being the smaller, and the slight 

 difference between 'them at the outset becoming 



° Figure 76. Probelocerus Lutheri. The 



constantly greater throughout the remaining inner whorls of an adult she " drawn ^ 



J ° ° ° transmitted light : showing the septa and 



history of the species. With the completion of 8iph0 125 

 the third volution the lateral saddle has become greatly increased in length 

 and breadth, the lateral lobe is considerably shortened and narrowed, and 

 the ventro-lateral lobe subacute. This form is very closely reproduced in 

 the anephebic conditions of Mantle. Pattersoni and in the mature state 

 of Mantic. simulator, in which the suture is characterized by the great 

 lateral saddle and the rounded lateral lobe. During the fourth volution 

 the tendency is toward the narrowing and sharpening of all the lobes and 

 saddles and their increase in actual size. Thus the fully mature suture as 

 expressed in the fifth volution, has a large, acute lateral saddle, a ventro-lateral 

 lobe of about the same shape and size, an acute but smaller lateral lobe, a sub- 

 acute, narrow, ventro-lateral saddle and an acute, moderately long ventral lobe. 



