THE AGE OF INVERTEBRATES OF THE SEA. 65 



These curious, and in the modern seas, exceptional shells, were 

 dominant in the Palaeozoic period. Upwarils of three thousand 

 fossil species arc known, of which a large proportion belong to the 

 Cambrian and Silurian, nine genera appearing in the Cambrian, 

 and no less than fifty-two in the Silurian. The history of these 

 creatures is very remarkable. The Lingulae, which are the first 

 to appear, continue unchanged and with the same phosphatic 

 shells to the present day. Morse, who has carefully studied 



'■''''••''Wr- 



..d 



Fig. 54. — Ltngula anatina. 

 With flexible muscular 

 stalk. Modern. 



Fig. 55. — Cambrian and Silurian Lingulae. n, 

 LiHgulella JWntihewi (Ha.ru). Acadian group. 

 d, LtHgula guadratn{yia\\). Lower Silurian. 

 c, Liujulella prima (Hall). Potsdam, d, 

 Linguiella antiqua (Hall). Potsdam. 



an American species, remarks in illustration of this, that it is 

 exceedingly tenacious of life, bearing much change of depth, 

 temperature, etc., without being destroyed. The genus Discina^ 

 which is nearly as old (Fig. 56), also continues throughout geo- 

 logical time. The genus Orthis (Fig. 57), which appears at the 

 same time with the last, becomes vastly abundant in Silurian 

 times, but dies out altogether before the end of the Palaeozoic 

 Rhynchonella (Fig. 58), which comes in a little later, near the 



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