RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 257 



tion, or if thus intombed they musl read) those sediments in the acci- 

 dental manner which has been described on preceding pages. 



It is reasonable to assume that at least as greal a proportion of ex- 

 tinct, as of living mollusks were destitute of protective shells, and 

 that as great a proportion were provided with only internal or other 

 imperfect shelly parts. This having been the case it is plain that a 

 large part of formerly existing mollusks can have no representation 

 among fossil remains, and that a large proportion of the others must 

 have failed of such representation. Still, mollusks as a class are so 

 generally provided with a complete shelly protection for their bodies 

 that these objects are among the most abundant and valuable fossil 

 remains of which the geologist makes use in his investigations. Their 

 value is enhanced above that of the remains of any other class as a 

 whole by the fact that so large proportion of them were denizens of 

 the waters in which were deposited the sedimentary formations which 

 are now characterized by them. 



Annuloida. — The existence in former geological time of others of the 

 Annuloida than those which constitute the class Echinodermata has 

 never been satisfactorily proved by the discovery of their fossil remains, 

 but there seems to be no reason for doubt that some such animals really 

 existed during at least a portion of that time. If such were the case 

 their failure to be represented by fossil remains was doubtless due to 

 the absence or imperfection of hard or fossilizable portions of their 

 bodies. On the contrary, only a few of all the living Echinodermata are 

 destitute of protective hard parts, which generally consist of a nearly 

 or quite complete calcareous spinous test that under favorable condi- 

 tions preserves the form of the animal after death. Most of them are pro- 

 vided with certain small internal hard parts, but no true skeletal frame. 



The abundance of discovered fossil remains of Echinodermata show 

 that the plan of their anatomical structure was essentially the same as 

 that of their living representatives. The living Echinodermata are 

 dwellers in marine waters, and it is presumable that the class has 

 always been confined to a marine habitat. In former geological ages 

 their representatives were abundant and varied, as is shown by their 

 often abundant calcareous remains which are found in many formations, 

 where they readily became intombed when those formations were in the 

 condition of sediments in the waters in which those Echinodermata 

 lived. 



Annulosa. — Of the five classes constituting the Annulosa remains of 

 the Crustacea are more likely than those of the others to be preserved 

 by sedimentary intombment, because all the members of this class are 

 provided with a more or less firm chitinous or calcareous covering for 

 their bodies, and because with few and comparatively inconspicuous 

 exceptions they are all of aqueous respiration and aquatic habitat. 

 Being of aquatic habitat the Crustacea of former geological periods are 

 H. Mis. 114, pt. 2 17 



