MOLLUSC A. 53 



Province III.— MOLLUSCA. 



Remains of the Testacea, or shell-bearing molluscous ani- 

 mals, are the most common of all fossils, and afford the most 

 complete series of "medals," or characteristic signs for the 

 identification of strata. The duration of types and species, as 

 a general rule, is inversely proportional to rank and intelli- 

 gence. The most highly organized fossils have the smallest 

 range, and mark with greatest exactitude the age of the deposit 

 whence they have been derived. But the evidence afforded 

 by shells, if less precise, is more easily and constantly obtained, 

 and holds good over larger tracts of country. 



The mollusca are soft invertebrate animals with une or 

 more nervous ganglions below the gnllet, whence nervous 

 chords proceed to form a collar round that tube, and also to 

 radiate to other parts of the body ; the ganglions developed 

 on these chords are scattered, in most, irregularly, and the 

 form of the body, in such, is nnsymmetrical. In a single class 

 (CepJialopoda) the muscles originate from an internal rudimental 

 cartilaginous skeleton : in the other classes they are attached 

 to the skin, or to the calcareous substance developed therein. 

 The blood is not red, and is usually colourless : the heart is 

 a muscular organ propelling the blood through a system of 

 arteries and veins, the latter being more or less in the form 

 of sinuses. The respiratory chamber, whether containing gills, 

 or organised as a lung, opens near, or receives, the amis ; the 

 intestine being bent usually forward to effect that relation. 

 Such is the grade of organisation of which the " Lingula flags" 

 and "Llandeils rocks" in the lower Silurian system have 

 yielded evidence. This testimony is by shells : most of the 

 mollusca are so protected. The shell is hardened chiefly by 

 carbonate of lime and may consist of one or two pieces, called 

 " valves ; " rarely of more. 



