EXmiiVSIC CHARACTERS PROGRESSIVELY MODIEIED. 349 



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

 puted Goniatitina^ 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 

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

 cies, all of which are Paleozoic. 



Quick Evolution of the Clymeniidae. — Of the Clymcniida;, 

 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 Clymeniidas, 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 Clymeniidai : 



"This extraordinary series shows the phenomena of quick evolution 

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

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

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

 complex sutures; and Gonioclymenidse, 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) yEquales, (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 (= ^quales in 

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



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



