I0I2 PRINCIPLES OF STRATIGRAPHY 



the decaying vegetable matter furnishing the carbon for coloring 

 the muds in which the organisms were buried. On the other hand, 

 it is not improbable that much of the carbonaceous material was 

 derived from the graptolites themselves. The general slight thick- 

 ness of these beds, and the fact that in successive beds the species 

 change, indicate a slow accumulation of the deposits in relatively- 

 quiet water. 



According to Ruedemann's observations (2t,), the young Diplo- 

 graptus, on leaving the gonophore, has already advanced into the 

 sicula stage, so that a free-swimming planula stage appears not to 

 exist. It is probable that this is true of most, if not all, graptolites, 

 and that hence the distribution of these animals is such as will be 

 accounted for by the vicissitudes which they met with as a floating 

 colony. 



Anthozoa. The Anthozoa are typically marine sedentary ben- 

 thonic animals, inhabiting the warmer waters of the oceans. A 

 large number are without hard supporting parts, and consequently 

 leave no remains, while others, probably the majority of Anthozoa, 

 secrete a calcareous or horny corallum, which is capable of preserva- 

 tion. Among the Actinaria, or fleshy polyps, a certain amount of 

 locomotion of a creeping or gliding nature is often observable 

 (Metridium, etc.), the individuals possessing this ability thus passing 

 from a normal sedentary to a vagrant benthonic life. A few forms 

 are also met with among the plankton. Occasionally epi-planktonic 

 individuals are met with, attached to floating al^se or timbers ; and 

 epi-vagrant benthonic individuals attached to moving crustaceans 

 are not unknown. The Madreporaria, or stone corals, are normally 

 sedentary forms, though they are not necessarily attached, but 

 may rest upon the sands. ( Fungia, some Porites.) 



Though the normal medium of the Anthozoa is salt water, a 

 few are known in brackish and even in tolerably fresh water. Cili- 

 cia rubeola is reported by the Challenger {Xo\. XVI, pt. II; 36) in 

 the river Thames in New Zealand; and Dana {g:i2o) states that 

 "... upon the reefs enclosing the harbor of Rewa (Viti 

 Lebu), where a large river, three hundred yards wide, empties, 

 which during freshets enables vessels at anchor two and a half 

 miles ofif its mouth to dip up fresh water alongside, there is a single 

 porous species of Aladrepora (M. cribripora) , growing here and 

 there in patches over a surface of dead coral rock or sand. In sim- 

 ilar places about other regions species of Porites are most common." 

 Several species of corals grow at the mouth of the Rio de la 

 Plata. 



Porites limosa flourishes in muddv water, and Astrea bower- 



