170 AN INTRODUCTION TO THE STUDY OF FOSSILS 



StrongyloceMrolus drdbachiensis is found in the deep waters 

 of Long Island Sound, in shallow tide pools north of Cape Cod, 

 and covering the rocks upon the Maine coast. It extends into 

 the Arctic Ocean and occurs also on the north Pacific coast. 



Fig. 68. — The Paleozoic sea urchin, Melonechinus muUiporns (Norwood and Owen) 

 which was very abundant in the shallow seas covering much of Missouri during 

 the St. Louis (Upper IMississippian) time. Dorsal view ; slightly reduced in size. 

 A, C, E, G, and / are the interambulacra ; B, D, F, H, and / the ambulacra. At 

 the tips of the interambulacra arc the genital plates, each with three pores ; the 

 ocular plates at the tips of the ambulacra are much smaller than the genitals. 

 (From Jackson.) 



