422 



THE GENERAL PRINCIPLES OF ZOOLOGY 



of the tetrabranchiate Cephalopoda, type Mollusca, is the cham- 

 bered or pearly nautilus. Fossil species related to this appear 

 in the Silurian rocks, and the Nautiloids, as they are called, 

 reached their greatest development in the Palaeozoic era. But 

 some genera that lived at this inconceivably remote time still 

 inhabit the earth. As an example, among the invertebrates we 

 may mention a genus of the Brachiopoda, Lingula (Fig. 398), 

 which occurs in the Silurian rocks and is found in our oceans 



Fig. 401. A, Dalmanites socialis, dorsal aspect; B, the same rolled up, lateral aspect; 

 C, ventral side of head <>t Phacops fecundus. c.sk, cephalic shield; e, eye ; f.c, fixed cheek; 

 f.s, frontal suture \gl, glabella; Ibr, labrum ; m.c, movable cheek; />, pygidium ; pi, pleura; 

 s.f.p, subfrontal plate; ///.thorax. (After Gerstaecker, from Parker and Haswell's Text- 

 book.) 



to-day. Of the vertebrates there is only one genus that has 

 survived, Ceratodus, the lungfish already mentioned. How 

 long this era lasted we cannot sav, but various attempts have 

 been made to estimate the number of years necessary for build- 

 ing up the various formations, and one of these, based on the 

 sedimentary rocks of North America, gives us 17,500,000 years 

 for the duration of the Palaeozoic era. If this period of time, 

 which is beyond human conception, is even approximate!}' cor- 

 rect, it is not surprising that so many different classes and 



