MUSEUM OF COMPARATIVE ZOOLOGY. 87 



Comparing the first septum with the corresponding septum of Dero- 

 ceras planicosta, it is noticeable that the ventral cell rind the lateral 

 lobes are deeper and narrower in the latter ; a characteristic liable, 

 probably, to considerable variation, since it accords with the larger size 

 of the neck of the ovisac, and the consequently greater spread of the 

 sutures in Goniatites diadema. 



The suture of the second septum differs far more from its representa- 

 tive in the young Ammonite. The ventral lobe of the latter, with its 

 siphonal fissure, is represented by an entire shallow lobe ; the superior 

 lateral cells have their counterparts in wide, shallow, evenly curved 

 sutures, unbroken by any superior lateral lobes. This comparison, and 

 the abrupt introduction of the superior lateral lobes in the third septum 

 of Goniatites diadema,* also show plainly that the superior lateral 

 lobes do not arise, as do the inferior lateral and auxiliary lobes, from 

 projections of the body, which appear first as single lobes upon the 

 edge of the umbilicus, subsequently becoming double (i. e. divided into 

 dorsal and ventral, vis-a-vis,) by the spreading of the sides of the 

 whorls. On the contrary, they are derived from the direct growth of a 

 lobe which bisects the large entire superior lateral cell, and divides it 

 into superior and inferior lateral cells. The inferior lateral lobes are 

 present in both. The outline of this suture in Goniatites diadema is 

 very like that of the adults of Nautilus atratus and similar species 

 among the Nautiloids which have sweeping lateral and shallow abdom- 

 inal curves. 



The next, or third septum of the young Goniatites diadema intro- 

 duces the superior lateral lobes as a minute angular indentation, not at 

 all similar to the rounded superior laterals of the young Ammonites. 

 The development of the sutures is more accelerated in the growth of 

 the latter than of the former, since here we have two septa with Nau- 

 tilian characteristics, in place of one >among the later-occurring and 

 more complicated Ammonites. When the young Goniatites diadema 

 is contrasted also with those of its own group, we find some interesting 

 resemblances. The Nautilini never develop superior lateral lobes, the 

 Uctrorsus group, Magnosellares of Sandberger, not until a very late 

 stage of growth, and in some varieties, as shown by the masterly 

 researches of the last-named author, these lobes are very slight, even 

 in the adults. I have not examined the young of other species and 



* Flate III, Fig. 4. 



