822 Report or tae Stare Gxoxoaist. 
Amphiclinodonta, Bittner. 1890. 
Shells exteriorly like Ampuiciina; interiorly articulation is aided 
by interlocking denticulate callous ridges and tubercles lying 
WZ adi 
rane 
Fia. 414.— Amphiclinodonta Zugmayeri, Bittner. (BITTNER). 
within the margins of the valves. Structure of the brachidium as 
in AMPHICLINA. 
Type, Amphiclinodonta Zugmayeri, Bittner. 
Distribution. Triassic. 
Rhynchonella, Fischer de Waldheim. 1809. 
The number of palzeozoic species which have been commonly 
referred to this genus, and, consequently, regarded as congeneric 
with the Russian upper Jurassic 2. lovia, Fischer, the type-species, 
is very great. To the most conservative student such an agglom- 
eration, presenting every variety of external configuration, must 
seem more like a hap-hazard and conventional association than 
a natural group. But it is, nevertheless, evident that features of 
internal structure, upon the variations in which generic dis- 
tinctions are usually based, are here most persistent. The 
crura, hinge apparatus and deltidial structure of R. lowia are 
features which were attained and became fixed in the Silurian 
period; the extreme pyramidal contour of that species, its smooth 
surface with few and faint marginal plications, is not, however, 
except in rare instances, reproduced among the paleozoic forms. 
What is thus true of the predecessors of 2. lowia is also, to a 
large degree at least, true of its living descendants. 
From a careful study of the structure of the ancient Rhynchon- 
ellas it has become apparent that slight variations from the type 
of interior possessed by 2. lowia are frequently of marked con- 
74 
ee 
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