BRAoHIOPODA. 915 
group, Disomva and Discrniscoa. All these forms are conven- 
iently grouped under the family term Discrwipz. 
In Aororrera, Conorreta, Linnarssonta, AcroTHELE and 
Ipuipxa the pedicle-aperture is persistently located at the apex of 
the pedicle-valve. This group of genera is one of very early date, 
for the most part contemporaneous with Parrrina, and the exist- 
ing evidence would indicate that it was not directly ancestral to 
the line of Trematis—-OrzicutomEa (Discrnipx). The incipi- 
ent formation of an internal foraminal tube is seen in several of 
these genera (AororrEeTa, AcrorneLr, Linnarssonia), and this 
feature attains its maximum in the true Sresonorrera of the 
Lower Silurian, where the foramen is still apical and the tube 
wholly internal. Hence Srpponorreta appears to be a normal 
termination of this line of descent. Scsizamson, in the compre- 
hensive meaning of the term ascribed to it in this work, has the 
pedicle-passage superficial, and in such shells as Schizambon fissus, 
Kutorga, and var. Canadensis, Ami, the condition of this passage 
is perfectly analogous to that of SreHonorrnra, the entire dif- 
ference being in the enclosure of the latter. In Scsizamson the 
fibers of the pedicle, extending through the foramen near the 
middle of the pedicle-valve, were directed toward the apex of that 
valve, and along the concave floor of the external pedicle-groove. 
The inner aperture of the pedicle-tube in SipHonorrera, corre- 
sponds to the “foramen” of Scu1zamson, and the outer aperture, 
or true foramen, of the former to the grooved umbo of the 
pedicle-valve in the latter. Hence in Scuizsmnon, thus considered, 
there is no evidence of a progress of the external aperture, or 
true foramen,'anteriorly beyond the apex of the pedicle-valve. 
These two genera are but slight departures from the same type 
of structure, but it would appear that this deviation took place 
during primordial times, as the typical Scuizamson (S. typicalis, 
Walcott) is a primordial fossil. The genus, Trematoxzoxus, 
Matthew (7. ensiynis, Matthew, type), appears to be another 
primordial representative of this structure, with the tubular 
enclosure of the pedicle more highly developed. Thus all these 
genera, from AcrorueLr to Scuizamson and SipHoNnoTReTa, possess 
an apical foramen, and the development both of the internal tube 
and the corresponding external groove has been a gradual one. 
They represent termini of slightly divergent series, consequently 
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