18S3L] 261 [Hyatt. 



the Goniatitinae except the Nautilinidae, and all the Ammoni- 

 tinae, and the funnel lobe is elevated upon it. 



An important factor in this classification is the dorsal suture, 

 and we find that the position of a genus may often be determined 

 in any given series by the peculiarites of this part ; whether it is 

 present or absent, and whether it has, or has not a small annular 

 lobe, or " spindle lobe," or a small saddle in the median line of the 

 dorsum. All the series, with few exceptions, begin in time with 

 arcuate forms which have dorsal saddles, and are succeeded by 

 nautilian shells with dorsal lobes, and then these acquire the 

 median annular lobes ; if they retain saddles on the dorsum, 

 the dorsal lobe ] invariably appears in descendants, but is apt to 

 be divided by a small saddle in place of an annular lobe. So 

 far as we know, the annular lobe appears in no species earlier 

 than the Devonian. 1 The " endosiphon," here spoken of for the 

 first time by that name, is the internal tube long known in Actin- 

 oceras, and lately demonstrated in Piloceras by Dawson, as hav- 

 ing its own proper walls. To this we can add a similar apparatus 

 observed in two good specimens of Endoceras, and also noted by 

 the author in some specimens of Sannionites.. 



Among Nautiloidea there are no series traceable directly to 

 arcuate forms after the expiration of the Carboniferous. This is 

 the common story, and we can see that the series must have arisen 

 very rapidly during the Paleozoic, branching out on every side 

 from the common ascending trunk of the straight and arcuate 

 forms. The same is true of the Ammonoidea in the Silurian, but 

 only one short series, the Nautilinidae, arises from the common 

 trunk of the straight cones. The close coiled shells of this series 

 become the stock form for the whole of the Ammonoidea. 



The Nautiloidea of the Mesozoic are all nautilian forms and 

 their genetic series do not present the rapid changes of form 

 observed in the Paleozoic, they are all close coiled and have as 

 observed by M. Barrande small umbilical perforations. 



This same statement applies also to the Ammonoidea, when near 

 their point of origin in the Silurian their forms are very quickly 

 evolved, but are much less quickly evolved after this period. 



The smaller genetic groups in the Paleozoic are distinguished 



1 These statements apply only to Nautiloids. See description of Goniatitinae 

 Nautilinidae, and Agoniatites. 



