CAMBKIAN FAUNA. 137 



repeated to the same extent in any other group, that while hosts of 

 genera, and even complete families of this order, which flourished 

 in the seas intermediate in time between the Cambrian period and 

 our own day, should have successively disappeared, a few individual 

 types seem to have survived from first to last, without having un- 

 dergone any essential modification of structure. Thus, the Lingula, 

 or Lingulella, of the Cambrian rocks is but very little, if at all, dif- 

 ferent from the existing Lingula, and it has indeed been considered 

 doubtful by some authors whether even specific characters could be 

 assigned to distinguish some of the earlier from the later forms, 

 separated by an interval of millions of years. The same persistence 

 of type is represented in the genus Discina. 



Side by side with these lower molluscan types, but appearing at 

 a somewhat later period, the Upper Cambrian, we find, as has al- 

 ready been stated, forms belonging to the highest order, the Cepha- 

 lopoda (Orthoceras, Cyrtoceras), another apparent contradiction to 

 the doctrine of progressive higher development. Considering the 

 group of the cephalopods by itself, however, we observe that its 

 earliest types belonged to the lower of the two divisions into which 

 the cuttle-fishes have been divided the tetrabranchiate, or four- 

 gilled order a division to which the somewhat later appearing, 

 and now probably disappearing, Nautilus also belongs. These 

 primitive cephalopods were succeeded in time by other members of 

 the same order Gyroceras, Nautilus, Goniatites until the Tri- 

 assic period was reached, when the first dibranchiate form, Belem- 

 nites, appears. From this period down to the close of the Meso- 

 zoic era both the two-gilled and the four-gilled forms occur in such 

 abundance that it would be almost impossible to state to which 

 group belonged the preeminence. But in the meanwhile a general 

 alteration and succession in the representative cephalopod type had 

 been taking place. The early forms already mentioned, Orthoceras, 

 Gyroceras, Cyrtoceras, and their allies, belonging to the family of 

 the Nautilidae, are succeeded in the Triassic period, where their last 

 traces (excepting Nautilus) are to be met with, by the members of 

 the more complicated group of the Ammonitidae, whose earliest 

 precursors (three or more species from the Carboniferous formations 

 of India, and a solitary species from the Carboniferous of Texas) 

 would seem to have been foreshadowed by Goniatites, a type struc- 

 turally intermediate between the Nautilidae and the Ammonitidae. 



