THE RECORD FROM FOSSILS. 1 03 



different from that of the modern animals. The latter appeared, 

 however, in the Devonian (3), and the ancient type disappeared with 

 the Paleozoic (2-4). The modern forms of star-fish appearing in the 

 Devonian (3) continued with little modification until the Jurassic (7), 

 when the more strictly modern families began to appear. 



Orphiuroidea (brittle stars). The brittle stars were quite abundant 

 in the Silurian (2), some of the genera being identical with those 

 living to-day. Their history has been of little special interest. They 

 have simply continued to expand slowly into the present condition. 



The Echinoidca (sea urchins). The sea urchins were well developed 

 in the Silurian (2) rocks, but were of a type quite distinct from the 

 modern forms (having more or less than twenty rows of plates). This 

 order of Palechinoidea continued to exist during the Paleozoic (2-4), 

 practically disappearing with its close. With the Mesozoic (5-8) the 

 modern urchins (with just twenty rows of plates) appeared, quickly 

 expanding, and reaching a profuse state of development in the 

 Jurassic (7) and Cretaceous (8). Since then they have been on the 

 wane. 



Holothuroidea (sea cucumbers). The sea cucumbers have either no 

 shells, or sometimes a few calcareous plates. As fossils they are 

 known only by specimens of these plates which are occasionally 

 found. No traces 4)L them are found earlier than the Carboniferous (4), 

 though the difficulty of determining their presence makes it not im- 

 probable that they existed at an earlier period. 



In general, then, the echinoderms were very early developed. All 

 of the classes, with the possible exception of the holothurians, were 

 found in the Silurian (2), and were as radically distinct from each 

 other then as they are now. The Paleozoic forms were on the whole, 

 however, quite distinct from the modern representatives. The 

 modern types appeared during the Mesozoic, and the group is at 

 present on the wane. 



MOLLUSCOIDEA. 



Under this head are included to-day two groups of animals, whose 

 position in the animal kingdom is unsettled, though they are certainly 

 related to each other. 



Brachiopoda. These include animals externally resembling bi- 

 valve mollusks. To-day they are few in numbers and do not form 

 a very important group, but in earlier times they were very abundant. 



