T>mlokjy I 



ItUAi H10I-ODA 



177 



conoontrio, xtn.itmii (growth lines) of the -urface. Mat when the mantle edges are 

 undulating or dentate the concontric L'niwtli lines are crossed by radiating strisc or 

 plication*. 



Tin- various parts of the sliell are conveniently brought out and illustrated in 



tlio following section on tonninology. 



TKRMINOLOOY. 



The designation of the various parts of the outline depends upon the 

 position in which the shell is placed. I shall adopt, because it is certainly the 

 mo-t convenient if not always the most natural position, the one in which 

 the beaks are placed uppermost and the hinge line nearly or quite horizontal. 

 The part in front of the beaks, toward which they are usually inclined, is 

 therefore considered as the anterior end, while that behind them, often much 

 the largest and widest, is the posterior. The upper edge is known as the 



lin-il or </",-sal margin, while the lower is called the basal or ventral. 



The length as given in the following pages always expresses the distance 

 I ctween the most prominent points (extremities) on the anterior and posterior 



Fig. 35. I and II, right ralvtt of I8CHYBODONTA (?) OVAI.l- I 'Irish and M A TIIKHIA RUOO6A Ulrith, 

 -<1 and divided iy linos to illustrate the tectfon on outline and dimensions. A-B, length; O-D, 

 rtor hlght: E K, |t.-rinr night: <i b, dorsal or cardinal margin: < J. anterior .-nd and margin; /, 

 ba il margin: g fc, posterior ,-n<l and margin. 



III, antero-cardinal uew of a small si f dinfamga eurta Whltfleld, from the upper part of 

 the Cincinnati group of Ohio; H, um bones and beginning of umbonal rldge; , eaoitcbeon; /, iunulr. 



I V. Hi.- two valTes of ao undeacrlbed sperles of Ortkodttma lying open In the shale and showing the 

 ligament at I ; middle beds of the Cincinnati tiniup at Om-innatl. 



V, dorsal view of an entire cut of the InU-rlor of OhMRNftnmM tgjnralu, a new geniu and specie* 

 fnim thi- upp.-r :- n.-innatl group of Ohio, hnwlng a shell gaping at U.th i-nds. 



VI. anterior vi,>w <,f liyttonyrhi.i radiata ( Aml<on\. it,, Hall), Illi. i shell with a 

 bysul <>|.-nitu'. This specimen is from Ciin-inn.it I, Ohio, and I* piullar In having thr right valv. 

 side of tlKiir.-) prcserTed as a cart of th.- .-xt.Ti.ir and th.- l.-ft vaUe as a cast .f n,. 



Is shown the pallial line (at p) running along the an: ,t iirnl. r th.- I- 



