Contributions to Invertebrate Palaeontology. 583 



tomed to examining tbem, which cannot be portrayed in a figure or 

 described verbally, but which often serve to detect, or at least aid in 

 detecting the true horizon of beds of rock which would otherwise 

 be left in doubt, and it appears necessary to have sonie means of 

 referring to or designating such forms when speaking of beds 

 characterized by them. The form under consideration resembles 

 those from the Chester limestone of Illinois, used in the description 

 given by Mr. McChesney, more closely than they do those from 

 the St. Louis limestone, given under the name P. tenuicosta by 

 Prof. Hall, which have a larger and more rounded beak and much 

 longer hinge-line; the striae, however, in its extreme fineness resem- 

 bles that of the St. Louis limestone specimens. 



Genus SPIRIFERA Sowerby. 



Spirifera (Maitinia) contractus. 



Plate XIII, figs. 17-19. 



Spirifera {Martinia) glaher var. contractus M. and W., Geol. Kept. III., vol. 2, 

 p. 298, pi. 23, fig. 5. 



Shell of medium size, broad ovate or globular in general form, with highly 

 ventricose or gibbous valves, and a short hinge-line with rounded cardinal 

 extremities. Ventral valve the most gibbous, with a large and strong incurved 

 beak ; cardinal area small, one-third or less than one-third as high as long, 

 divided in the centre by a rather wide fissure ; hinge-line less than half the 

 width of the shell below, the cardinal slopes strongly and abruptly rounded ; 

 centre of the valve deeply impressed by a moderately wide, subangular mesial 

 sinus. Dorsal valve nearly orbicular, moderately convex from side to side ; 

 beak small, slightly tumid, projecting slightly beyond the cardinal line; 

 mesial portion somewhat strongly elevated to form the mesial fold, which is 

 of moderate width, not strongly defined at the margins, and does not extend 

 above the middle of the shell. Surface of the valves smooth to the naked eye, 

 but under a magnifier is seen to be marked by fine, obscure, radiating lines 

 and by transverse lines of growth. 



All the specimens seen are exfoliated to a greater or less degree, 

 so that the real surface has not been seen. The surface strife, seen 

 by the aid of a lens, is too strong and distinct not to be a sur- 

 face character, as they are readily felt by the hand, although not 

 readily visible to the unassisted eye. The shell does not attain a 

 very large size, no specimens examined exceeding one and three- 

 eighths inches in length, by a transverse diameter of about one and 

 one-fourth inches. The shells are somewhat variable in form, being 

 proportionally more or less elongate than the measurements above 



