THE AGE OF INVERTEBRATES OF THE SEA. 63 



Cyslideans continue unimproved to the end. Tlie Crinoids 

 culminate in the Mesozoic, and are not known to give origin to 

 anything higher. The star-fishes and sea-urchins commer.v.3 

 independently, before the culmination of the Crinoids, and, 

 though greatly increased in number and variety, still adhere 

 very closely to their original types. 



Fig. 49. — Plcurocys.iites 

 sqnamosns. L. Silunan. 

 After Billings. 



Fig. 50. — HeterocrhiHS 

 simplex (Meek). One c\ 

 the least complex crinoids 

 of that period. Lower 

 Silurian. 



Fig. 51. — Body of Gly^it}- 

 crinus. Lower Silurian. 



The great sub-kingdom of the Mollusca, including the bivalve 

 and univalve shell-fishes, makes its first appearance in the 

 Cambrian, where its earliest representatives belong to a group, 

 the Arm-bearers or Lamp-shells (Brachiopods), held by some to 

 be allied to worms as much as to mollusks. The oldest of all 

 these shells are allies of the modern Lifiguice (Fig. 54), some oi 



