334 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
ductus.” Spines have not been observed in the Ordovician homeomorph, and a 
strongly lamellose shell like that of Productorthis is not a characteristic feature 
of the Carboniferous productids. Internally, of course, the resemblance to “Pro- 
ductus’ is very remote except for the hinge. 
The delthyrial cavity of Productorthis is deep, and the muscles occupying it 
do not make a pattern readily recognizable as orthoid. The adductor field is very 
wide and occupies most of the delthyrial cavity. On each side of this track occurs 
a long, slender, and deeply impressed scar that can now only be interpreted as 
the place of attachment of the diductor muscles. Schuchert and Cooper regarded 
these scars as belonging to the adjustor muscles, but the Virginia specimens, 
particularly the one illustrated on plate 56,B, figures 14 and 17, make it neces- 
sary to revise this view. It will be noticed that a narrow pallial trunk extends 
from the anterior end of these slender scars. These trunks are undoubtedly the 
vascula media which always originate at the anterior ends of the diductor scars. 
Adjustor marks were not definitely seen in any of the specimens, and it is as- 
sumed therefore that if these muscles were present they were situated on the 
sides of the delthyrial cavity and left no visible marks. In the Virginia material 
the dental plates are not strongly marked because they are partially obliterated 
by callus deposited in the umbonal cavities. In old specimens the dental plates 
are not visible. 
The interior of the brachial valve of Productorthis is as distinctive as that of 
the pedicle valve, but the orthoid characters are more clearly retained. The 
brachiophores are short and distinctly orthoid but are partially buried in callus 
that surrounds their anterior ends to form a cuplike socket. The cardinal process 
is large with a long oblique shaft that extends into the interior. The myophore 
face is narrowly elliptical and bears a sharp, slender median carina. The postero- 
dorsal extremity of the cardinal process is partially or wholly covered by a small 
elliptical or circular plate which is undoubtedly a modified chilidium. The ad- 
ductor field of the Virginia specimens, like their European relatives, are divisible 
into 6 individual scars. The posterior pair are short and wide. The anterior 
group is divisible into a larger median pair and a smaller outer pair. The median 
ridge is low and extends for the length of the adductor field which may or may 
not extend as far as the middle of the valve. 
Prior to the present study Productorthis was only known outside of Europe 
in South America, and Kozlowski has speculated on its absence from North 
America. The interesting feature of the North American specimens is their large 
size, which is about twice that of the usual European representatives. So far the 
genus is known only from Virginia and Alabama where it is a rare fossil at all 
its localities. Two specimens only are known from the Arline formation in the 
form of a species resembling P. eminens (Pander) but much larger. The same 
species is known from the Little Oak formation of Alabama and the Botetourt 
formation of Virginia. The reefy masses of the Effna limestone in Virginia have 
yielded interesting species of this genus which seem to be restricted to this pe- 
culiar environment. 
