ON THE STRUCTURE OF THE SHELL IN THE 



GENUS ORTH1S. 



By James Hall. 



It is generally pretty well known among palaeontologists, at the pres- 

 ent time, that the genus Orthis, as constituted by Dalman, contains 

 heterogeneous material; and that the species do not form the well 

 characterized natural group sometimes claimed for them. 



Leaving out of consideration the two species first named by the 

 author of the genus, which are marked with an ?, the remaining spe- 

 cies exhibit a considerable variety of external form and of internal 

 marking, which characters alone are sufficient to distinguish them gen- 



erallv from one another. 



The general aspect of the shells constituting the genus, as described 

 by its author and extended by subsequent writers, is a sub-circular or 

 sub-quadrate form ; valves sometimes nearly equally convex, while in 

 other examples one valve may be flat or concave. This latter feature 

 may affect either the ventral or the dorsal valve. Both valves are fur- 

 nished with an area, though this character is often but slightly devel- 

 oped in the dorsal valve ; the opposite valve is furnished with wider 

 area and open triangular fissure for the passage of a pedicil. The 

 hinge line is straight, usually shorter than the width of the shell. 

 The surface is striated or plicated, and the general aspect of nearly all 

 the forms is so similar that they have been grouped together, gener- 

 ally, and by the best authors, without hesitation. 



The most conspicuous external difference is between a finely striated, 

 and a coarsely plicate surface. These differences are often accompa- 

 nied by another distinguishing feature. The coarsely plicate forms, 

 among the American species, are usually what are termed resnpinate 

 shells; the dorsal valve being the more convex and the ventral valve 

 flat or concave and sometimes sinuate in front, but still carrying its 

 conspicuous area and foramen. On further examination we find that 

 many striated species are resupinate, or have the dorsal valve the 

 more convex. An examination of the interior of the shell in all these 

 forms shows that the muscular impression in the ventral valve is 

 strongly defined, distinctly bilobate, limited at the margins by a 

 strong ridge or elevated lamella, usually interrupted or non -continu- 

 ous in the front. (These forms are chiefly of lower or middle Silurian 

 in their geological range.) . 



On comparing other forms of the genus where the valves are nearly 

 equal, or where the shell is plano-convex, the more convex valve is the 

 [Sen. Doc. No. 53.] 10 



