I \VU.I ll;i;.\N< IIIATA. Ml 



posterior position of the heaks ;ire features that render tin- identification of tin- 

 specif- nnii-ually .M~\ . 1 h.'-ii.itrd to gay whether it should be regarded aa nearer 

 ( '. nititl.i or those ovate >hells, like < in which the larger side is undeniably 



tin* posterior. 



formation <m<i ! iii!-, Thin small h>'ll <K-< it tiutulM>re In certain layen of th in 



thlrtl of tin- Tr.'iii..n shale* at St. I'aul, M -..I .it nor lix-mlltl.- in Hi,- 



i !.i\. r may IK- .-mi.; I i.\ separated vah it- nf UK- mtvrlnr. 



'I'll' illlnit one at th<- two locallUe* iir-t merit ii>n<il, Imi In (J.xKlhuo and 



i .until* tmtlfenmN example* are the nil.'. In central Kentucky the upeclwt U uccanlonally 



met with in th<- ModlolcMlon Utl- ..f th. Tr.ni. .11. 



Mu*. Reg. No. 86t7. 



CTBNODONTA KKCUNDA Hall. 



I'l.XTK XI. II HiH. -. 



>'<myatfe,-un,ln M.M.I.. 1882. Geol. Sur. WU.. vol. I. p. 66. (Figured, l.ut nut dcwrlbed.) 



II small, '-' nun. to 13 mm. in length, rather ventnco.se, tntnsversely ovate or 

 oliscuri'ly sulirhoniKoiilul in outline, \sith the umbones rather prominent and full, 

 .UP I tin- I'l-ak- iiiciirveil. directed slightly forward and situated about one-third of 

 the li'ii^th liehiml the anterior extremity; base usually a little prominent in the 

 mi. Idle, somewhat straightened, or at any rate less convex in the posterior than in 

 the anterior half; posterior end narrower than the anterior, the outline sloping 

 forward rapidly above the produced lower part and merging almost gradually into 

 the post-cardinal margin; antero-cardinal outline more or less distinctly concave; 

 posterior umbonal ridge rounded. Surface marked by very fine, regular concentric 

 -t ri.r aud strong wrinkles of growth, crossed by delicate radial lines, the network 

 thus formed requiring a magnifying lens to make it plainly visible. The radial 

 lines, however, are not often preserved. 



The majority of the specimens seen are casts of the interior, mostly in an excel- 

 lent state of preservation. As a rule, they are marked by a limited number of obscure 

 concentric furrows. The muscular scars and pallia! line are always faintly defined 

 Hinge plate rather narrow, arcuate, nearly two-thirds as .long as the shell, with about 

 eighteen denticles in each valve; denticles very small under the beaks, where the 

 series seems also to have been interrupted by a small space; on each side of the 

 beaks they become larger gradually and at the same time assume an oblique direc- 

 tion, the upper ends of the teeth being turned away from the beaks. 



Three specimens, illustrating slight variations, have the follow-in- dimensions: 

 Length, 10.5, 11.0 and 13.0 mm.; bight, 7.0, 8.0 and Inn .; thickne. t..'.. 6.5 and 

 6.8 mm. 



This very common shell is certainly di-tinct from C. kvatn, C. nitida and C. K#- 



i, the anterior end being narrower and in two cases also shorter, while thr 



