844 



NORTH AMERICAN INDEX FOSSILS. 



operculum — the lid or cover closing 

 the opening of various shells, etc. In 

 Hydrozoa, see I., 21 ; in gastropods, 

 see L, 584, and Fig. 920 ; in cephalo- 

 pods, see aptychus and anaptychus. 



Opisthodetic — in pelecypods, referring 

 to the ligament when present only 

 behind the beak, as in Venus. 



Opisthogyrate — curved backward. Ap- 

 plied to the umbos of pelecypod shells, 

 as Nucula and Trigonia. 



Orals — in crinoids, the five interradial 

 plates surrounding the mouth (see 

 II., 491, and Fig. 1810), 



Oriskanian — upper part of lower De- 

 vonic. 



Oriskany beds — lower Devonic of New 

 York, Pennsylvania, etc. 



Orr formation — upper Cambric of Utah. 



Orthaulax bed — middle Oligocenic of 

 Florida. 



Orthoceracone — a straight Nautiloid 

 cephalopod shell. 



Orthoid — shaped like Orthis (see I., 

 185, 4). 



Osage group — upper division of lower 

 Mississippic of Mississippi Valley. 



Osculum — an opening ; in sponges, the 

 large terminal opening (see Fig. 21). 



Osgood beds — Niagaran of southern 

 Ohio and Kentucky. 



Ossicle — a little bone ; also one of the 

 small calcareous particles forming the 

 skeleton of some echinoderms, as the 

 star fish. 



Ambulacral ossicle — for use in aster- 

 oids, see II., 571, and Fig. 1910. 

 Vertebral ossicle — for use in ophiur- 

 oids, see II., 570. 



Oswayo group — upper Mississippic of 

 southwestern New York. 



Ouray limestone — Devonic and Missis- 

 sippic of Colorado. 



Oviform — egg-shaped. 



Paddles — in eurypterids, the large or 



last pair of thoracic legs. 

 Paget formation — upper Cambric of the 



Canadian Rockies. 

 Pali — in corals, narrow vertical plates 



inserted between the columella and 

 the inner ends of the septa (see Fig. 

 168). 

 Pallial — pertaining to the mantle of 

 mollusks and brachiopods. 

 Pallial line — the line on the interior 

 of the shell of mollusks, marking 

 the attachment of the mantle. In 

 pelecypods, see Fig. 476. 

 Pallial sinus — in many pelecypods, 

 the reentrant angle at the posterior 

 end of the pallial line ; it marks the 

 point of attachment of the muscles 

 of the siphon (see Fig. 476). In 

 brachiopods, one of the main blood 

 vascular canals ; see vascular mark- 

 ings, also I., 174). 

 Palmars — in crinoids, the third series 

 of brachial plates, lying above the 

 axillary distichals (see Fig. 1907, P). 

 Palmate — resembling a hand with fin- 

 gers spread. 

 Palpebral — pertaining to the eyelids. 

 Palpebral lobes — eyelids or supra- 

 orbital extensions from the fixed 

 cheeks of trilobites (see Figs. 1542, 

 1557). 

 Pamelia limestone — upper middle Ordo- 



vicic of New York. 

 Pamunkey formation — lower Eocenic 



of Atlantic coast. 

 Papilla — a minute, cone-shaped projec- 

 tion. 

 Papillose — containing or covered with 



small rounded projections. 

 Parabasals — in crinoids, the second (up- 

 per and outer) cycle of basal plates ; 

 usually called basals. 

 Parabolic — like a parabola, a curve 

 formed by the intersection of the sur- 

 face of a cone by a plane parallel to 

 one of its sides. 

 Paradoxides beds — middle Cambric of 

 Atlantic coast, exclusive of basal bed. 

 Paragaster — the large central cavity of 



a sponge (see Fig. 21). 

 Paraptera — see II., 424. 

 Parasite — an animal which lives either 

 upon or at the expense of another. 

 Parivinculum — in pelecypods, applied 



