PALEOZOIC TIME — CAMBKIAN. 487 



of T. S. Hunt (page 73), probably indicating, as has been suggested, the 

 presence of much phosphatic material in solution in the seawater. 



The Lamellibranchs are the lowest of Mollusks, and the species were very 

 -small. The Gastropods were very small, and mostly of the Patella-like 

 symmetrical, non-spiral species ; but with these occur species of Platyceras, 

 having a short spire, and some of Pleurotomaria, a Paleozoic genus of coiled 

 species that continue on through later time. 



The Crustaceans are either species of the lower division of the class, the 

 Entomostracans, or are Isopod in relations. 



Smallness of size is not, however, a universal feature. The Pteropods, 

 among Mollusks, were much larger than the modern species of the tribe. 

 The Trilobites even of the Lower Cambrian comprise species as large as 

 living Crustaceans. The Ostracoids are generally larger than those of recent 

 times. 



The most prominent exception to low-grade features in the fauna is that 

 ■of Trilobites, which have nearly the perfection that belongs to the typical 

 Isopod. Their primitive character is, however, marked in the multiplicate 

 structure of the thorax and its limbs, and in the fact, observed by Beecher, 

 that each of the thoracic legs has a natatory appendage. 



Embryonic jyrecursor lines fail. — The Lower Cambrian species have 

 not the simplicity of structure that would naturally be looked for in the 

 earliest Paleozoic life. They are perfect of their kind and highly specialized 

 structures. Xo steps from simple kinds leading up to them have been dis- 

 covered ; no line from Protozoans up to Corals, Echinoderms, or Worms, or 

 from either of these groups up to Brachiopods, Mollusks, Trilobites, or other 

 Crustaceans. This appearance of abruptness in the introduction of Cambrian 

 life is one of the striking facts made known by geology. But, as is often 

 urged, this appearance of abruptness is believed to be due to defective records. 

 In some regions there are thick strata in the Cambrian below the lowest fossi- 

 liferous beds representing a long lapse of time, besides others in the Archaean, 

 of whose life nothing is yet known. Again, species without shells or stony 

 secretions make no fossils, and can leave no record ; and it is for this reason 

 that we know so little of Cambrian Worms, all that remains being the holes 

 or tracks they made. 



Further : the Lower Cambrian rocks are often hard slates and grits, and 

 the heat, or heated moisture, or siliceous solutions, that hardened them would 

 have tended to dissolve away calcareous shells. The shells of phosphatic 

 kinds, as the Lingulse, Discinse, and the tests of Trilobites, would have 

 suffered least. Prom this last fact it follows that resistance to solution, not 

 predominance in number, may, in many cases, have determined the relative 

 proportions of the species of fossils. These are sources of uncertainty 

 demanding consideration. 



The Olenellus beds have been made theLoioer Cambrian. But they are 

 not necessarily the lowest. Por if strata should be found containing no 

 Trilobites, but only Worms, the lower types of Brachiopods, Ostracoids among 



