Report of the State Geoloqisi. 



473 



Sfpvrifer arsnosw, to which are attached a number of the young 

 of Oyrtvna rost/rata y in their original position. The specimen 

 shows thai attachment by the pedicle in spiriferoid shells 

 brought the cardinal area close and flat against the surface of 

 adherence, the pedicle evidently having been extremely short. 

 Jn Mich close attachment, obstructing, from the first, additions to 

 to the posterior margin of the shell, may be suggested an 

 explanation of the origin of the cardinal area and extended 

 hinge. 



In concavo-convex species, as of Strop heodonta, Product its, 

 Plectambonites, etc., the pedicle was evidently atrophied very 

 early in the history of the individual. Some such shells, as 



Fir;. 30. — Spirifer asper. 



Fig. 31.— Young of Cyrtina rostrata attached by 

 the face of the cardinal area to the surface of Spiri- 

 fer arenosus. Oriskany sandstone, Cumberland, Md. 



Hafinesqdina and Lept^na, at times retain at maturity, a slight 

 external evidence of a foramen, or the foraminal passage may 

 even remain open as a minute puncture, but in this condition no 

 sufficient number of muscular fibres could have passed through 

 it to have supported the shell. Such shells, in becoming freed 

 from the surface to which they were attached, would fall upon 

 the sea-bottom, the heavier or pedicle-valve down, so that virtually 

 this still acted as the attached valve. It will readily be under- 

 stood that the contour of these concavo-convex shells is a neces- 

 sary result of an obstruction to the marginal growth of the 

 brachial or concave valve, by the more rapid growth of the 

 pedicle valve. 



1891. 60 25 





