304 GEOLOGICAL BIOLOGY. 



largest species of the Niagara or Silurian is not more than an 

 average for the whole range of size. If the size is expressed 

 on a scale of lo, i representing the smallest and lo the 

 largest, the range in the Silurian is about i to 4. There is 

 a general tendency to increase in the size of species of the 

 genus from their beginning to the Carboniferous. The Silu- 

 rian species average about 3 on such a scale, the Devonian 

 species 5, the Carboniferous 7, and the size of the Mesozoic 

 species would average about 3. 



The species which contain the larger individuals for their 

 period are generally more abundant in numbers. There is 

 evident adaptation of size and abundance to conditions of en- 

 vironment, for certain deposits contain abundant and large 

 representatives of a particular species, while other deposits 

 contain but few and small individuals. The character B, then, 

 is evidently in its evolution purely extrinsic, the species adapts 

 itself to environment, and in each race the adaptation is 

 greater with advance of time up to the Carboniferous, where 

 the whole race deteriorates, and in most species becomes ex- 

 tinct, only a few surviving, and those having some specially 

 developed characters. 



C. The DcltJiyrium and Deltidium. — The delthyrium is the 

 opening through which the peduncle passes for the attach- 

 ment of the shell, and its covering is the deltidium. In its 

 early stage the young shell was always attached, and the del- 

 therium was open. In some species there was very plainly a 

 gradual closing of the fissure by d. pscudodcltidmui, a covering 

 of shell growing over the fissure from beak downward. In 

 others, there is this pseudodeltidium with a slight foramen 

 permanently running through it (see Fig. 82, A, B, D, and 

 E, d). 



In others there is a permanent open fissure ; at least, no 

 calcified covering is present in the adult. The presumption 

 is that there was variation in the length of time the individual 

 was attached, some species becoming free at very early periods, 

 others remaining attached throughout life. If we express this 

 character mathematically, i referring to attainment of free 

 state very early, 10 permanently attached, we find among the 

 species of the lowest period of the Silurian (Niagara and cor- 



