50 



Report of the State Geologist. 



In a specimen of Mantic. intvmescenstTomihs lower upper Devonian limestone 

 at Bredelar, Westphalia, the protoconch appears to l>e somewhat larger, about 

 .9 mm. in length and its convexity as viewed from the side very great, more 

 pronounced than in any of our specimens. We shall have occasion to direct 

 attention to the fact that, within comparatively restricted limits in these 

 cephalopods, the protoconch varies greatly in form, in minor generic divisions, 

 if not among commonly accepted species of the same genus. Slight variations 

 of form in the protoconch are associated with, and probably in part responsible 

 for wider divergences in the later characters of the shell. 



Hel&tive asymmetry of the Protoconch. Repeated observations seem to 

 indicate that the protoconch is asymmetrical, not in itself but with reference 

 to the first whorl of the shell. That is to say, the nepionic shell described a 

 revolution not along, but at a very slight angle to the dorso-veritral axial plane, 

 and one of its convex and protuberant lateral extremities therefore protrudes 

 farther than the other. This may not always be the case, at least in some 

 instances it is difficult of detection, but in the majority of instances examined 

 it is clearly shown. We are not able to state whether this prominence of the 

 protoconch is invariably the greater on a given side, but it thus far appears to 

 be a peculiarity of the dextral side of the shell. A minute inclination of the 

 shell in drawing will obscure or aggravate this asymmetry, and we have, in 

 consequence, redrawn the majority of the figures here presented. It is import- 

 ant to understand that this asymmetry is not inherent in the protoconch, but 

 rather in the direction of revolution at the commencement of the nepionic stage. 



Relation of the Protoconch to Umbilication. From the much greater 

 diameter of the protoconch than that of the nepionic whorl it is clear that 

 umbilication w as an unavoidable necessity for at least an early period in the 

 life of the species. 



Progressive Modification of Form. — The Nepionic Shell. The 

 nepionic stage of shell growth is clearly delimited. The 

 embryonic condition terminates with the first septum, but 

 this septum is formed before the full reduction of the 

 convexity begun in the protoconch. This is also clearly 

 show n in several of the associated species, Mantic. nodifer, 

 Probeloceras Lutheri, etc. Thus the expanded portion 

 of the nepionic whorl which bears the second septum, is a 

 Figure n. side view of pro- determinate fraction of that whorl. The nepionic shell 



toconrh and nepionic shell, - 1 



showing the broad round makes slightly more than one-half, sometimes nearly three- 



varix with which the latter o J J 



terminates. Taken from f ()lir ths of a volution and is characterized by its smooth 



specimen in advanced " 



growth. x25. surface and its slight diameter which at its close is palpably 



