Hyatt.] 308 [April 4, 



among the Ammonitinae. Silurian forms are notable as having 

 sutures with undivided ventrals of the simplest type, shallow 

 primitive outlines to the first pair of saddles, primitive first lat- 

 eral lobes and dorsal sutures with broad lobes or saddles, but no 

 annular lobes, and the funnels are machrochoanitic. There are 

 exceptional genera, but these are very rare. Devonian forms 

 have the large magnosellarian lateral saddles, and transitional 

 characters in the forms, septa, and outlines of the sutures, which 

 are advances towards the Ammonitinae. The ventral lobes when 

 divided are apt to be very broad and the siphonal saddles large. 

 In the Upper Devonian and Carboniferous there is a tendency to 

 narrow the ventral lobe and produce smaller siphonal saddles, and 

 divide the first pair of saddles and also the magnosellarian sad- 

 dles into smaller lobes and saddles. Forms having marginal lobes 

 and saddles, which make near approaches to the Ammonitinae 

 occur only in the Carboniferous. 



The abrupt appearance of the Goniatitinae in the later Devo- 

 nian and earlier Carboniferous formations of North America, and 

 the absence of radical forms of the group of Nautilinidae in the 

 Silurian indicate that they were migrants from European sources 

 and not autocthonous. 



Nautilinidae} 



This family includes forms with gyroceran and nautilian whorls, 

 and sutures with simple lateral lobes either throughout life or 

 until a late stage of growth. There is a dorsal lobe in the 

 impressed zone, which is due to the involution of the dorsal 

 saddle suture, which is present in the larva. There is no 

 annular lobe except in the more aberrant forms. The ventral 

 lobe is a true funnel lobe in the lower species, becoming a wider 

 ventral lobe in the higher species, as in Pinnacites. The funnels 

 are long and tapering, and in species with- approximate septa 

 seem to be continuous, though really ellipochoanoidal. The shells 

 are banded with transverse striae, but otherwise smooth, and the 

 apertures have a ventral sinus, which may be in exceptional transi- 

 tional forms like Mimoceras quite small, but are usually large. 



1 On p. 256 mention of the Magnosellaridae as having undivided ventrals and ot the 

 fact that the dorsal saddles were confined to the lower forms of Nautilinidae was acci- 

 dentally omitted. 



