MESOZOIC ERA: THE AGE OF REPTILES 



525 



dant, though not diversified, in the Jurassic, at which time the 

 crinoids attained their greatest size and beauty. The stem of one 

 has been traced seventy feet without reaching either end. The 

 " head " in some individuals (Fig. 488 A) is as large as a feather duster 

 and similar to it in appearance. The genus to which these large 

 specimens belong (Pentacrinus) is still found in the West Indian 



seas. In America the class appears 

 to have been rare throughout the era. 

 Although the structure of the Meso- 

 zoic and Tertiary crinoids differs 

 markedly from that of the Paleozoic, 

 perhaps the most conspicuous external 

 difference lies in the great develop- 

 ment and subdivision of the arms and 

 the relatively small body (calyx) of 

 the later type. All 

 Paleozoic crinoids 

 were attached to the 

 sea bottom by stems, 

 and this was also 

 true of the great 

 majority of the Ju- 

 rassic genera, but a 

 few free-swimming 

 forms began then and 

 have continued to 

 the present. In these 

 unattached forms, 

 the animal begins its existence fixed to the bottom by a stem, as did 

 its ancestors, but later becomes free. 



Sea Urchins (Echinoids). — A new type of sea urchin (Fig. 489 

 A-C), which had a few forerunners in the later Paleozoic, soon en- 

 tirely replaced the older type. One marked difference between the 

 two groups lies in the number of rows of plates forming the " shell," 

 which was variable in the old, but in the new was constant. Early 

 in the era, a fivefold symmetry (Fig. 489 A) was the rule, but later 

 a twofold or bilateral symmetry characterized the greater number 

 of species. Sea urchins with club-shaped spines (Fig. 489 A) were 

 abundant in the Jurassic and Cretaceous. Inconspicuous, and for the 

 most part rare throughout the ages of the Paleozoic, sea urchins hid a 



Fig. 488. — Mesozoic crinoids: A, Pentacrinus fossilis ; 

 B, Apiocrinus parkinsoni (without arms). 



