472 



Forty-fifth Report on tee State Museum. 



the brachial valve may be compared to an operculum closing it ; 

 and in this fact is probably one efficient reason for the differences 

 of size in the two valves. 



Figs. 26 and 27. — Rhynchonella acuminata. (Davidson.) 



In Disoinisca the pedicle is a short plug extending at right angles 

 to the horizontal plane of the valve ; and here the lower or pedicle- 



J valve is flat and the upper conical, while in Acro- 



[ treta and CoNOTRETA,where the pedicle was probably 

 g^ longer and more flexible, the lower or attached 

 valve is highly conical and the upper nearly flat. 

 As a general rule shells closely attached by a pedicle 

 at this high angle to the plane of the brachial valve 

 have a tendency to circular or subequal peripheral 

 ,_ growth (Discina, Discinisca,Orbiculoidea, etc.), and 

 p e d icTe n v& ifvl a similar tendency is manifested in shells attached 

 loZeviorsi^e and by solid cementation, not only in Crania, Craniella, 

 (w^lco™.)^ 6 *' - etc., which are hingeless genera, but also such gen- 

 era as are provided with a hinge, when attached, evince a 

 spreading or ostrean form of growth. Articulate shells, where 



the pedicle has maintained its func- 

 tion during a considerable portion 

 of the period of adolescence (some 

 species of Spirifhr,Cyrtina, Athyris, 

 Strophomena, Lepivena, etc.), but 

 later becomes atrophied, so that the 

 animal must have dropped Prom its 

 support and fallen upon the sea 

 bottom, are found to combine an 

 elongate form with considerable 

 breadth of hinge, and often with 

 area. In the Oriskany sandstone of 

 Cumberland, Matvland, lias been found a fragment of a valve o\' 



24 



Fig. 29. 

 shells of 



KKHY). 



A cluster of young and old 

 Ditinisca Utmcllosa (Sow- 



a highly developed cardinal 



