58 



Report <>f the State Geologist. 



Figure 28. Manticoceras Pattersoni. The 

 course of the sutures from the ananepionic 

 shell to maturity. 



that the hitherto broad ventral lobe is divided by a low, sharp angle directed 

 backward and which forms at the same time a ventro-lateral lobe and a ventral 



saddle of equal size. The introduction of this 

 modification is not accompanied by any other 

 change on the external suture, but concomi- 

 tant with it is a crenulation of the heretofore 

 simple dorsal suture producing the dorsal 

 saddle and subdorso-lateral lobe. All of our 

 evidence points to the fact that this radical 

 modification in the course of the suture com- 

 mences simultaneously and, as during its re 

 maining history no important further change 

 occurs, it must be regarded as making the com- 

 mencement of a distinctive growth period. 

 We shall presently observe that the new 

 departure in this structural feature is contem 

 poraneous with modifications in other differ- 

 entials. During the later progress of the 

 suture its form is progressively modified by 

 e. length of the ventro-lateral lobe, which in 

 the adult is long, acute Avith slightly convex sides, having a lanceolate outline. 

 The great lateral saddle grows proportionally larger, shows more and more 

 distinctly its characteristic inclination toward the umbilicus and in its ulti- 

 mate stage its outer or convex curve develops a shouldered appearance. 



The lateral lobe gradually narrows, maintains its rounded extremity, even 

 into the anephebic substage, in which condition the suture has throughout the 

 character of that of Man tic simulator. This lobe is angled in normal mature 

 conditions. 



Gontow of Septa. The first septum is evenly concave outwardly, with 

 an abrupt increase about the sipho. With growth, ultimately this con- 

 cavity, though not wholly lost, is greatly modified by the multiplication of 

 lobes and saddles on all margins. A section along the plane of coiling will 

 give a series of septal lines in which the earlier have a pronounced convexity 

 throughout, but this is gradually encroached upon by a concavity above, 

 between the ventro-lateral lobes, and another below at the long dorsal lobe. 



Sipho. Traces of the inceptive portion of the sipho are clearly retained 

 on some pyrite internal casts of the protoconch. The extent of this caecal 

 tube is evidently considerable and its ventral position very distinctly indicated. 



tli 



ra] 



•id 



increase in size, 1. 



