22 PALAEONTOLOGY OP THE UPPER MISSOURI. 



This family includes the genera Productus, Straplialosia, Aidosteges, and Chonetes. 

 It commences in the Silurian rocks, and ends with the Permian, being a strictly 

 Palaeozoic group. 



Genus CHONETES, FISCHER. 



Synon. Pectenites, Pectunculttes, and Pectunculus (sp.) of tlie early authors. 



Pecten, USE, Hist. Kuthenglen, 1793, pi. xvi, fig. 10-11 (not Miiller, 1770). 



Jlysterolites and Terebratulites (sp.), SCHLOTH. Petref. 1820, 256, pi. xxix, fig. 3. 



Productus (sp.), J. de C. SOWERBY, Min. Conch. IV, 1823 (not 1815). BDCH (sp.), Abh. der K. Akad. Wiss. 



1841, 33. 



Leptcena (sp.), GOLDF. Germ. Tr. de la Beche's Geol. 1832, 523 (not Dalm., 1828.) 

 Chonetes, FISCHER DE WALD. Oryckt. Mosoou, 1837, 134. KONINCK, An. Foss. 1843, p. 206. ; Id., Monogr. 



M^moires Soc. Liege IV, 1847, 1. GEINITZ, Grundr. der Verstein. 1846, 517. HALL, Palseont. N. Y., 



II, 1852, 64. 

 Strophomena (sp.), HALL, Geol. 4th Distr. N. Y. 1843, 72 and 180. 



Etym. x"" 1 ) a lit*' 6 DOX - 



Examp. Chonetes Dalmaniana, DE KONINCK. 



Animal unknown. Shell transversely semicircular, concavo-convex, compressed, 

 greatest breadth usually on the hinge line ; area common to both valves. Ventral 

 valve convex, usually depressed along the hinge; area generally broader than 

 that of the other valve, and inclined more or less back over the hinge angular 

 and armed along the margin with a row of tubular spines; foramen partly or 

 entirely closed by a false deltidium. Dorsal valve concave ; foramen replaced 

 by a prominent bifid or trifid cardinal process. Surface of both valves sometimes 

 nearly smooth, but generally ornamented by concentric marks of growth, and fine 

 radiating strise, or rarely large plications. 



Hinge with two cardinal teeth, located one on each side of the foramen of the 

 ventral valve, and fitting into corresponding pits in the other. Interior of both 

 valves provided with a narrow more or less distinct mesial ridge, extending at 

 right angles from the hinge, part of the way across, between the muscular im- 

 pressions. Scars of the cardinal muscles in the ventral valve, ovate and somewhat 

 oblique ; those of the adductors very small, and placed between the impressions 

 of the cardinal muscles and the mesial ridge. Dorsal valve with four small 

 adductor muscular impressions, and two short linear hook-shaped vascular scars. 

 Interior of both valves more or less granulated. 



This genus was introduced during the Lower Silurian Epoch, and continued its 

 existence at least to the close of the Carboniferous, in which it attained its maxi- 

 mum development. 



Chonetes mucronata. 



(PLATE I, Fig. 5, a, b, c. d, e.~) 



Chonetes mucronata, MEEK & HAYDEN, Proceed. Acad. Nat. Sci. Phila. 1858, 262. 



Compare C. Smithii, NORWOOD & PBATTEN, Jour. Acad. Nat. Sci. Phila. 1857, 24, pi. ii, fig. 2. 



Shell rather large, compressed, semicircular, having its greatest breadth on the cardinal border, which is extended 

 into mucronate angles. Surface ornamented by a few subimbricating concentric marks of growth, crossed by very 



