GLOSSARY. 



Sigmoid — curved in two directions like 



the letter S. 

 Silicification — the process of combining 

 or impregnating with silica, or the 

 state of being so impregnated (see I., 

 4). 

 Simpson formation — middle Ordovicic 



of Oklahoma. 

 Sinistral — left-handed ; applied to the 

 reversed coiling of some gastropod 

 shells, as Physa (see Fig. 1204, and 

 compare with the dextral coiling of 

 Limncea, Fig. 1203). 

 Sinuate — wavy, winding. 

 Sinuosity — a notch or incision forming 



a wavy outline. 

 Sinus — an elongate down-bending or 

 depression. 



Hyponomic sinus — the single median, 

 marginal, concave bend on the ven- 

 ter of some cephalopods ; it indi- 

 cates the position of the hjrponome 

 (see IL, 2z)' 

 Median sinus — in some brachiopods, 

 the central and usually the largest 

 depression extending from the beak 

 to the front of the shell and usually 

 upon the pedicle valve. 

 Sipho — referring to siphuncle ; a com- 

 bining word (see endosipho-lining 

 and preseptal sipho, II., 20). 

 Siphon — in some pelecypods, one of 

 the two, more or less tubular pro- 

 longations of the posterior mantle 

 edges ; the ventral or branchial si- 

 phon gives entrance to water carry- 

 ing food and oxygen, the dorsal or 

 anal siphon gives exit to the water 

 carrying waste matter (see L, 362). 

 In cephalopods, the fleshy, hollow cord 

 prolonged from the base of the man- 

 tle passing through a rounded aper- 

 ture in each septum, and extending 

 to the inner side of the first or initial 

 chamber. 

 Siphonal — pertaining to the siphon. 

 Siphonal funnel — the tubular contin- 

 uation of a septum around the si- 

 phon. When this is prolonged 

 backwards, as in most nautiloids, it 

 is spoken of as retrosiphortate, 



when prolonged forward, as in 

 most ammonoids, it is prosipho- 

 nate. It forms part of the siphun- 

 cle (see also II., 20). 

 Siphonal lobe — the lobe in the su- 

 ture of an ammonoid shell which 

 corresponds in position to the si- 

 phuncle ; the ventral lobe. 



Siphuncle — in cephalopods, the seg- 

 mented horny or calcareous, tubular 

 wall secreted by and surrounding the 

 fleshy siphon. It consists of the 

 siphonal funnels and connecting 

 sheaths (see Figs. 1240, 1254, d, 

 1282). 



Skaneateles shale — middle Devonic of 

 New York, 



Slickensides — polished or striated sur- 

 faces on rock due to motion under 

 great pressure. 



Slit — for slit and slit-band in gastro- 

 pods, see I., 583, and Fig. 830. 



Snowbird formation — lower Cambric of 

 southern Appalachians. 



Sockets — see dental sockets. 



Somite — one of the segments, either 

 visible or ideal, of an arthropod or 

 vertebrate body (see Fig. 1542). 



Spatulate — shaped like a spatula; 

 spoon-shaped. 



Species — one of the smaller divisions in 

 classification. 



Specific name — the second of the two 

 or three names applied to a single 

 fossil. 



Spence shale — middle Cambric of Utah, 

 Idaho. 



Spergen limestone — middle Mississippic 

 of Mississippi Valley. 



Spheroidal — somewhat like a sphere. 



Spicule — a minute spike or dart. In 

 sponges spicules vary much in shape 

 from a single needle-like form to a 

 very complex body of many points. 



Spine — in gastropods, see I., 583. 



Spiniform — spine-like. 



Spinose — full of spines or thorns. 



Spinulose — spine-bearing. 



Spiracle — in blastoids, the five or ten 

 round or slit-like openings surround- 



