Nuouiid^.] LAMELLIBEANCH1A.TA. 583 



Group V is probably the most distinct of all these sections. It is certainly the 

 least variable and the easiest to recognize, the Astarte-like form of the shells alone 

 being suflficiently diagnostic. The subrostral interruption of the hinge denticles is 

 very distinct and the point is often marked bya sort of pit, quite undefined, however, 

 that may have lodged an internal cartilage. Nucula may really have been evolved 

 from this 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. As it is, 

 C recurva is nearer Nucula than it is to C. nasuta, but several species of the levata 

 section approximate that genus 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 Nucula 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 Birdseye and Black River limestones as One) 

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

 tennesseensis and nasuta, the gibberula group by all of its species except C. carinata, 

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

 subrotunda, the recurva group by G. campressa, and the sixth group by C. logani. 

 Each 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 

 is 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 Birdseye 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 form'ation abounds in Gastropoda and Cephalopoda. 



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

 of Ctenodonta and certain forms of recent genera like Neilo, Malletia and Sarepta, three 

 nuculoid genera, and Axincea and other Arcidce. If this work was not already growing 

 beyond the limit? alloted to it, I would gladly give the results of these comparisons 

 here fully, but under the circumstances I am obliged to restrict myself to a few 

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

 the G. nasuta 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 

 Nucula. Certain Cretaceous species of Axincea (e. g. A. sulplanata StoUczka) are strik- 

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



