1888.] NATURAL SCIENCES OF PHILADELPHIA, 229 



width of fifty specimens is nine millimeters, much below normal ; 

 specimens very much larger occur in other horizons. Here it is 

 generally perfectly preserved in all its details ; the mesial fold is 

 more sharply defined, and the depressions on each side of the fold 

 relatively deeper than is usual Avith this species. The depauperate 

 condition of this, and in fact of all the brachiopods from the same 

 horizon, is suggestive of an environment, at the time these animals 

 lived, that was extremely unfavorable to the full development, and 

 to the attainment of a normal size that would be rendered possible 

 by a more congenial habitat. 



Chonetes laevis nov. sp. (Plate XII, figs. 3a, 3b.) 



Shell small ; much wider than long ; transversely semi-elliptical ; 

 the cardinal line as long as the greatest width of the shell, or often 

 slightly extended beyond the lateral margins. Ventral valve con- 

 vex, with no indication of a mesial sinus ; beak not prominent ; 

 cardinal area rather narrow but Avell denned centrally, becoming 

 linear toward the extremities ; foramen moderately wide ; cardinal 

 margin bearing from four to seven oblique spines on each side of the 

 beak. Dorsal valve flat or very slightly concave ; with no mesial 

 fold. Surface of both valves apparently perfectly smooth ; but 

 under a magnifier it is seen to be marked by numerous fine concen- 

 tric striae, and more j^rominent, often somewhat imbricated, lines of 

 growth ; these are sometimes crossed by fine nearly obsolete radia- 

 ting strise. 



Length 7 mm. ; breadth 12 mm. 



This species is found in the superimposing black shales of coal 

 No. 3 at Des Moines; and is associated with Chonetes mesoloba, 

 Productus muricatus, and the minute gasterpods hereafter mentioned. 

 The glabrate character, and the absence of a mesial fold and sinus, 

 as is constant iu all eight of the specimens found, forms a marked 

 contrast with the associated congeneric forms, in which the radiating 

 strise are unusually sharp and well defined ; and also with the other 

 carboniferous forms of the same genus. This species is closely allied 

 to, and perhaps identical with, the form described by Geinitz' as 



1 Carbon formation und Dyas in Nebraska, 1866, p. 60. 

 Chonetes glabra; but this name, however, was preoccupied by Hall 

 in 1857, for a species from the Upper Helderburg. 



Streptorhynchus crenistria Phillips. 



Spirifera crenistria Phillips, 1836. Geol. Yorks., II, p. 216, pi. 

 ix, fig. 6. 



