BRACHIOPODA. 
167 
leaving, from youth to maturity, a triangular opening extending the entire 
width of the area. In Volborthia, Iphidea, and Acrotreta, the ridge or furrow 
indicates the modification of the surface made by the aperture. In Acrothele, 
the posterior slope is again less defined, and the progress of the foraminal scar 
not distinctly marked upon its surface. In these features the passage to 
Siphonotreta is imperceptible, while in Schizambon, the close ally of Siphon- 
otreta in most respects, the position of the aperture is in front of the beak, 
and its advance with growth leaves between it and the apex a conspicuous 
furrow crossed by concentric lines of accretion. This we regard as an extreme 
of development in the character of the pedicle-passage, a point which is not 
passed by these shells. 
The enclosure of the pedicle-slit, leaving only a circular perforation for the 
passage of the arm of attachment, appears to have been unaccompanied, as far 
as our evidence reaches, by any essential variations in internal conformation 
in the genera Volborthia, Iphidea, Chonotreta and Acrothele. In Acrotreta 
there is an elevation or mammiform swelling about the internal opening of 
the sipho, which necessarily gives the passage the character of a short tube. 
In Linnarssonia and Discinopsis this internal swelling becomes more extremely 
developed in lateral extension, while in Siphonotreta it is expressed in a greater 
longitudinal development, producing a well developed tube. The muscular 
impressions of this group of genera are so imperfectly understood that it is 
impossible to correlate them satisfactorily, but their biological relations, as 
far as expressed in the characters of the pedicle-passage, may be represented 
by the following diagram: 
Obolella—Leptobolus—Paterula— 
I I 
Schmidtia Schizobolus 
(Discinolepis) 
This scheme is not intended as an expression of the derivation or geological 
succession of these genera. Leptobolus and Schmidtia follow Obolella in the 
[Schizopholis. 
/Ijinnarssoniax 
,'Siphonotieta—Scliizamboi 
