LAMELLIBRANCHIATA. 583 
Nuculide.] 
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 sufficiently diagnostic. The subrostral interruption of the hinge denticles is 
very distinct and the point is often marked by a 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 C. compressa, and the sixth group by C. logani. 
Each group again is as sharply marked in these first species as itas 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 formation 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 Nelo, Malletia and Sarepta, three 
nuculoid genera, and Avinea and other Arcide. If this work was not already growing 
beyond the limits 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 C. 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 Axinea (¢. g. A, sulplanata Stoliczka) are strik- 
ingly similar to the C. pectunculoides section, the only difference of real consequence 
