I.AMKU.IU: \\rlll.\T\ 

 <! | 



proluilily the ui'>>t di.stim-t of all the-. - 'tns. It is certainly tin- 

 least variable and tht> >a>iest to recogni/e. the A-tarte-like form of the shells alone 

 l>eiiig sufficiently diagnostic. The slintral interruption of the hinge denticles is 

 very distinct and th- point is often marked by a sort of pit. <|iiite undefined, however, 

 that may have lodged an internal cartilage. .\U'-K/H may really have been evolved 

 from thU type, since it would have required but a slight modification of the hinge, 

 a depression or lengthening of the form, and a filling of the umbones. Ay it i-. 

 ( '. n-rnn-,1 is nearer \nruln than it is to C. nasuta, but several species of the levata 

 section approximate that p-nns even more closely, so that we are obliged to regard 

 the balance of the evidence to be in favor of the levata group, unless both the 

 groups have contributed to make \ucula as now understood. 



Of Group VI only C. logani is well known, so we cannot say much about affin- 

 ities. The species are all Trenton, and their general aspect is quite different from 

 the other groups. 



It is an interesting fact that all of these sections are represented already in the 

 lowest geological division (considering the Hirdseye and Black River limestones as one) 

 in which the genus makes its first known appearance; the nasuta group with the species 

 tf*nestens\s and nasuta, the gibberula group by all of its species except C. caritwta, 

 the levata group by at least five species, the petunculoides group by the specie? 

 "nl'i-otunda, the recurva group by C. compressa, and the sixth gronp by C. lognni. 

 Kadi group again is as sharply marked in these first species as it is at any subse- 

 quent time: nor have we any evidence to aid us in deciding which of the six groups 

 i- the most like the primitive stock. It is evident, therefore, that a long line of 

 forms of this type must have existed in the ages preceding the Hirdseye of which we 

 now have no knowledge whatever. The same remarks apply almost equally well 

 to the other families of Lamellibranchiata, and one of the most remarkable facts in 

 paleontology is the almost total absence of the class in the Calciferous, especially 

 when we consider that that formation abounds in Gastropoda and Ceptuflopoda. 



I have carried on a number of very interesting comparisons between the species 

 of ( 'tenodonta and certain forms of recent genera like Neil. Malletia and Sareptn, three 

 nuculoid genera, and Asin-ra and other Arriii-i . If this work was not already growing 

 beyond the limits alloted to it, I would gladly give the results of these comp;in-nh- 

 here fully, but under the circumstances I am obliged to re-tri.-t myself to a few 

 general remarks. The three nucnloid genera mentioned are very similar indeed to 

 the r. nasuiii group of species, the first and second differing chiefly in having a sinu- 

 ated pallial line, while the third has an internal cartilage pit beneath the beaks like 

 .\ncula. Certain Cretaceous species of Asimm (f. >/. A. sulplanntn Stnli. . /..n .ire strik- 



similar to the C. pectunculoides section, the only difference of real consequence 



