86 BULLETIN OF THE 



splendid material accumulated by Professor Agassiz, in doing this with 

 considerable certainty for the Arietidae, Liparoceratidre, Cycloceratida;, 

 and somewhat less completely for several other families of typical 

 Ammonites. 



Goniatites has the first septum very similar to that of the typical 

 Ammonites, but on the ventral side a deep siphonal fissure occurs, ow- 

 ing to the near approach of the conical prolongation of the siphonal 

 ccecum to the ventral side.* The sutures of the first septum follow 

 the cone in two specimens of Goniatites diadema, but in both the neces- 

 sarily violent removal of the shell probably carried away the outer side 

 of the siphon. 



Barrande's observations show that this frequently occurs in the adult, 

 when the whorls of the Silurian Goniatites are torn apart in order to ex- 

 pose the ventral side. The broken edges of the upper part or neck of 

 the siphonal coecum, in both the specimens alluded to, as in the one fig- 

 ured,f make continuous sutures along the abdomen from the first to the 

 second septum, and this could hardly be accounted for under any other 

 conditions. 



A comparison of sections of this part of Deroceras planicosta and 

 Goniatites diadema J show that in both the same conical prolongation 

 exists, and has about the same relation to the shell. In the former, 

 however, the cone ceases to be an integral portion of the siphonal coecum, 

 and the invariable continuity of the suture of the ventral cell is due to 

 the manner in which the first septum bends inward to form the floor of 

 this part only, instead of being more deeply inflected to form the cone, 

 as in Goniatites.§ Both are normally entire, but in Ammonites alone, 

 owing to the structure of the cone, is this apparent in all specimens. 

 The suture of the second septum in Goniatites diadema is continuous ; 

 the neck of the siphonal coecum not being so close to the shell, it is 

 not easily obliterated, and the dark line of the suture is readily traced 

 across the siphonal area. This septum has entire outlines, a shallow 

 broad ventral lobe, and superior lateral cells, with narrower, but still 

 very well marked lateral lobes near the embryonal umbilicus. || The 

 dorsal sides of neither of these septa were observable on account of 

 the opacity of the specimens. 



* Plate III, Figs. 3, 5, 6. § Plate II, Fig. 1, 1 e. 



t Plate III, Figs. 3, 5, 6. || Plate II, Figs. 3, 4. 



t Plate I, Figs. 3, 4. Plate III, Figs. 5, 6. 



