I.AMKI.I.II.K \N> II1A I \ 

 .UP.) 



II. C. </ifili,-rnl(i i 



sholls usually >hort, thick. with vcr\ *tronj.'ly defined nniM-iilar impressions; 

 hinge strongly Kent, with the series of detiticlps interrupted beneath the beaks; 

 teeth -traiu'ht or curving outward. 



Spi-cie*: 1/ililifrnln Salt-r. ,;,nt>--i<-t-i Salter. attyelti Hillings, i-arinnta Ulrich. ///imw/r- 

 i I Iricli. /.IH/./ 1'lrich. 



III. ' '. l-Tnta group. 



Shells tonally of ovate form, rather thin, with muscular scars moderately dis- 

 tinct; .Ifhtn-Ir- converging inward, forming a continuous series in the Trenton 

 species. In the Hudson Kiver forms, however, the series is more or less interrupted 

 l>y the development of a small and undefined pit just beneath the beak. 

 Species: levata Hall. ln<i<-it',irmis Hall, ul-nj>t>i Billings, nitidn Ulrich, medialis Ulrich. 

 scqfiif'H I 1 rich, socialis Ulrich, harixvillensis Safford, danvillensis, n. sp., retrosa 

 I'lrich. fiHstri'itn Ulrich, albertina Ulrich, simulntrix Ulrich, fuvtida, n. sp., 

 madisonensis Ulrich. tecunda Hall, mlrin I'lrich. tnnn<lnl<i. \\. sp., perminnin 

 I'lrich. Hiini-uliformis Hall, ? hilli Miller. 



IV. C. pectunndmdes grotty. 



Shells subcircular, compressed-convex, beaks very small; hinge plate strong, 

 regularly arcuate; teeth in a continuous series. 



Species: sitbrotunda Ulrich, circulnris, n. sp., pectunculoides Hall, cingulata Ulrich, 

 pulcMl.i Hall. 



V. C. recurva group. 



Shells high, the lower half semicircular, the upper subtriangular; hinge plate 

 rather strong, bent at nearly a right angle, the (?) anterior part convex, the (?) 

 posterior concave; denticles in two distinct series, arranged transversely on the 

 plate and therefore at widely different angles on the two parts of the hinge. 



,,,,ni>,f-.^,i I'lrich. urtt.irtil'ormix Salt. .|.-./i'.i I Irich. ,ilf,i Hall, "bli 



Hall. '^,y/^jL, /+*** ? 



. 

 J 



VI. C. logani group. 



Thin giltbous shells, with subcentral large beaks; muscular -MI- f.iint: hinge 

 hut little arcuate, tin- ilmticles in continuous series, bent inward. 



ies: logani Salter. <lubin Hall. ; /il>l>s<i Hall. ? anita Hall. 



Group I. the typical section of the genus, seems to be strictly confined to Lower 



Silurian deposits and embraces the largest known representative- of tin- family 



/' 'iroiip II ap; '-von more rr-tricti-<l in it> ran^e. l>ein^ unknown 



