114 INTRODUCTION. 



dental valve, and corresponding sockets excavated on each side of the cardinal prominence 

 already described. In the dental valve, a small longitudinal ridge divides the muscular 

 impressions situated on either side, the cardinal muscles probably occupied the greatest 

 space, the adductor lying on either side close to the mesial ridge. In the socket valve a 

 blunt medio-longitudinal ridge divides the quadruple impressions of the adductor muscle, 

 which forms on either side two oval scars from between which (in some specimens) two 

 short vascular impressions proceed in an outward oblique direction, when turning back- 

 wards and inwards, they terminate at some distance from their origin. The interior of 

 the valves is covered with minute granulous asperities, arranged in longitudinal lines ; 

 animal unknown, probably free or attached in the young by fibres issuing from the 

 fissure. 



Obs. In 1837, Fischer de Waldheim proposed the genus chonetes,^ but did not 

 characterise it sufficiently ; and it was only after the publications of M. de Koninck' 

 and De Vernueil,^ that its value became known, and the appellation generally used. 

 Several authors have remarked that the shells composing this section bear affinities, 

 both to Productus and Leptana. Professor M'Coy, in 1852,* makes Chonetes a sub- 

 section of Leptcsna. The outer resemblance it bears to many of the species of the 

 last-named genus is obvious ; but internally there exist differences which remove it from 

 the Strophomenidce, and unite it by family ties to the Productidce, and in particular to 

 Strophalosia : these are the disposition of its quadruple adductor scars and the reniform 

 vascular impressions, which I was so fortunate as to discover in a specimen from Nehou 

 (PI. VIII, fig. 200), and although the latter character is unobservable or indistinctly 

 marked in the generality of Chonetes, I have been able to trace its existence in several 

 species. Chonetes also possesses the double area, articulated hinge, and tubular spines 

 of the Productida, and although the last character cannot be claimed as of generic 

 importance, it is at any rate common to all the Productida, and wanting in the 

 StrophomenidcB. 



Chonetes should therefore be placed first or last among the Productida, and near 

 to the Strophomenidce, as it is a genus or sub-genus which in a measure links the two 

 families together. 



We must refer our readers, for detailed descriptions, &c. to the excellent monograph on 

 this genus, published in 1847, by M. de Koninck : the occurrence of spines only on the 

 outer edge of the area of the dental valve, distinguishes the species of this section from 

 those of the other Productida, a circumstance of great convenience in the present case. 



Geol. range. — Chonetes first appeared in the lower Silurian age, and continued through 



' Oryctographie du Gouv. de Moscow, p. 134. 

 ' Description des Anim. Foss. du Terrain Garb, de Belgique, 1843. 

 ' Russia and Ural Mountains, vol. ii, 1845, 



^ British Pal. Fossils, 1852. American authors have often erroneously placed Chonetes among their 

 Strophomenidce. 



