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 Acrotiiele, 

 the posterior slope is again less defined, and the progress of tlie 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 : 



Discinopsis 

 rKutori^Mnji 1 | 



Olmlclla— Leptobolus— I'.ntei-ula— — - | — Volliorthi.'il /I.innai-ssonia\ 



I I IscliizopliiilisJ I —Acrotreta ;Siphonotret.a— Schizamljoii 



Sclirnidtia Schizobohis Ipliidca J | Acrotiiele / 



[ Chonotreta 



(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 



