582 THE PALEONTOLOGY OF MINNESOTA. 
LNuculide. 
above the top of the Trenton. Group III is by far the largest section of the genus 
both in its specific and individual development. It may justly be called the nuculoid 
section, since not only the general expression of the shell is decidedly like Nucula, 
but its internal characters likewise approach those of that remarkably persistent 
type more closely than is the case with any of the other groups here defined. I 
think that the evidence indicates very strongly that Nucula was developed from this 
stock. As is well known, that genus is distinguished from Ctenodonta chiefly in 
having a small but well defined internal cartilage pit immediately beneath the 
beaks. Now, although in the Trenton forms of Group III the hinge denticles form 
a perfectly continuous series, this cannot be said of the Hudson River species. In 
many, if not all, of these, namely, the series of teeth are more or less distinctly 
interrupted beneath the beaks by the incipient development of an at least similar 
pit. So far as it is possible to say, true species of Nucula occur in the Devonian, so 
it is but natural to assume that the missing links between them and the Ctenodonta 
levata group of species are to be found in the intervening Upper Silurian deposits. 
But here we meet with an obstacle in the fact that none of the Upper Silurian shells 
that have been referred to Ctenodonta (Tellinomya) and Nucula*, with the possible 
exception of Tellinomya curta Hall, of the Clinton group, belong to the C. levata 
section. It does not, however, follow that such species did not exist, though we 
must admit that it is a strange, if not a significant fact that they have not yet been 
found. Still, the significance of their absence is lessened when we consider that the 
Upper Silurian deposits throughout are relatively poor in remains of Lamellibran- 
chiata. It is also to be remarked that the forms which have occurred belong chiefly © 
to families widely different from the Nuculidw. It is possible that the Devonian 
genera Palwoneilo and Nuculites also came from this stock, such a development being 
faintly indicated by C. fecunda and C. nuculiformis; but taking all the characters 
into consideration, and the direction of the variation that may be followed into the 
lower divisions of the Upper Silurian, Clidophorus seems to me a more likely ancestor 
for those genera. 
Group IV may be a departure from the C. recurva group, but, as it seems to me 
to be a more primitive type, I would rather consider the relation as reversed. The 
only objection to the latter arrangement may be removed at any time, since it is 
nothing more than that C. compressa, a typical species of the recurva group, has been 
found somewhat lower in the Trenton formation than the earliest known member 
of the pectunculoides group. 
*Very little is known of the hinge of the Up. Sil. species that have been referred to Tellinomya by Hall and others. 
so that we are justified in doubting that they really belong to the genus. Those known to possess a denticulated hinge 
are much more like Palwoneilo than Ctenodonta. 
