264 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Si 



D 



Fig. 116. — Bactrites gra- 

 cilior Clarke, from the 

 Naples beds (Upper 

 Devonian) of New 

 York. A, a nearly en- 

 tire shell ( X I). B, an 

 ideal longitudinal sec- 

 tion showing septum 

 (sp.), suture (su.), and 

 siphuncle (si.). C ( X 

 20), apex of shell show- 

 ing protoconch (pr.) 

 and, where the shell is 

 broken away, the su- 

 tures (su.). D, upper 

 surface of a septum 

 showing siphuncle (si.). 

 (After Clarke.) 



-hites, meaning stone). Sutures simple 

 except for the small ventral lobe. Si- 

 phuncle ventral, sub-marginal. 



1. Sketch (a) side view showing ventral 

 lobe ; (b) top view ; label sutures, ventral 

 lobe, siphuncle. 



2. Why is this not included with the sub- 

 order Nautiloidea ? 



Muensteroceras (Fig. 117). 



Mississippian. 



Shell coiled into a thick disk. Umbihcus 

 moderately wide, with angular margin. 

 Sutures with angular lateral lobe and 

 broad lateral saddle, and with a narrow lobe 

 on the venter broken by a small notched 

 saddle. 



All shells with the above type of simply 

 angular sutures were formerly classed as 

 goniatites (from Greek gona, a knee). 



1. Sketch view showing coils, noting su- 

 tures, lobes, saddles, umbilicus; indicate 

 the ventral side. 



2. How was the umbilicus formed ? 



3. Sketch an entire suture from venter to 

 umbilicus. 



4. What is a suture ? 



5. What determines the dorsal and ven- 

 tral sides of a shell ? 



Placenticeras (Fig. 118). Cretaceous. 



Shell coiled into a thin disk. Suture 

 complex, made up of numerous serrated 

 lobes and saddles (the ammonitic type of 

 suture) ; the third lateral lobe is the deepest, 

 after which there is an abrupt decrease. 



