Review of Geinitz on the rocks and fossils of Nebraska. 175 
nearly equal convexity of their valves, usually smaller number 
of cartilage cavities, and generally less elongated form in pro- 
portion to the length of the hinge line, are characters which, 
when viewed collectively, indicate generic differences. The 
most important distinction, however, is found in the hinge teeth, 
which although variable in both groups, differ in that those on 
while those in front of the beaks range obliquely forward and 
downward, instead of forward and upward, asin Gervillea. Again 
Bakevellia has the anterior muscular scar proportionally larger, 
Dimyaria, though it belongs to the Avicula group near Gervillia. 
een or sixteen years past, to clearly separate these groups. 
These are the proportionally larger size of the posterior ear in 
Aviculopecten, instead of the reverse, and its broad Pterinea-like 
Under the beaks. Even Woodward, who was the most ex- 
Temely conservative of all modern conchologists, in regard to 
generic distinctions, not only separated these shells from the ge- 
hus Pecten, but even placed them doubtfully in the family Avie- 
.“®, No well informed conchologist would at the present 
time refer to the genus Pecten, as properly restricted, any Paleo- 
IC species,* : 
€is also behind the present state of conchological science 
In placing in the genus Arca forms like the so-ca striata 
Schlot. (sp.), with the posterior hinge teeth linear, elongated 
ua Tanging parallel to the cardinal margin. These belong to 
the genus Macrodon of Morris and Lycett. Even Woodwai 
Separates this group from Arca, but without sufficient reason in- 
cludes it as a subgenus under Cucullea, There are no true Ar- 
“88 In the Paleozoic rocks. . ae fess 
nother unnatural mingling of distinct ae is his refereneg 
of typical Ledas (properly Nuculang,) to the genus Nucula. 
* Prof. Agassiz, who has gone over the whole family Pectenide with great care, 
and - usual thoroughness se not, if reierag arts porn anes 
