EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 349 



Silurian and are dominant in the Devonian, and the undis- 

 puted Goniatitinae are not continuous beyond the Carbonif- 

 erous. Sagiceras and like forms are Triassic, and are inter- 

 mediate between this and the true Ammonite type. The 

 Goniatitidae (v. Buch, emend. Zittel) contain about 300 spe- 

 cies, all of which are Paleozoic. 



Quick Evolution of the Clymeniidae. Of the Clymeniidae, 

 about 30 species are known all from the Upper Devonian. 

 When, however, the character of the suture is made the chief 

 means of classification, we find a considerable range of modifi- 

 cation in the Clymeniidae, and of the other characters: the 

 shape of body whorls, rounded, angular, tuberculated, etc., 

 and amount of involution of whorls, all indicate great modifi- 

 cation, so that authors have classified even this special little 

 group of forms into many genera. Hyatt proposes 3 families, 

 with 9 genera in all, based upon the minute studies of 

 Giimbel. Hyatt remarks, regarding the Clymeniidae : 



"This extraordinary series shows the phenomena of quick evolution 

 in three series of forms. Cyrtoclymenidse, with a series beginning with an 

 Arcestes-like form, and passing through discoidal and compressed to quad- 

 ragonal forms ; Cymaclymenidae, a similar parallel series, but with more 

 complex sutures; and Gonioclymenidae, also a similar series, but with more 

 involute forms than the last, and the sutures becoming Ammonitic, with 

 median ventral lobes and saddles, divided by a pair of marginal lobes." * 



When we compare this series of suture-lines with those 

 of a single Goniatite, at different stages of individual growth 

 (Fig. 112), the evolution may be expressed as a case of rapid 

 acceleration, with some variation added. 



Classification of the Goniatites. The attempt to classify the 

 Goniatites by their sutures has resulted in various systems, in 

 each of which the particular form of the mature suture-line 

 has been the criterion of classification. 



Beyrich proposed six groups, which he called (i) Nautilini, 

 (2) Simplices, (3) ^Equales, (4) Irregulares, (5) Primordiales, 

 (6) Carbonarii. 



Sandberger made a more minute analysis, based upon the 

 form of the lobes and saddles making up the suture. His 

 nomenclature is: (i) Linguati, (2) Lanceolati (= ^Equates in 

 part of Beyrich), (3) Genufracti (= Carbonarii Beyr.), (4) 



* See " Genera, Foss. Ceph.," p. 313. 



