CHONETES OF THE HAMILTON GROUP. 135 



the margin. The radiating striae are crossed by extremely fine concentric 

 elevated striae. 



The cardinal margin shows two and rarely three spines on each side 

 of the centre, which are abruptly bent outwards so as to lie nearly 

 parallel to the hinge-line, and the outer one extending much beyond the 

 cardinal extremity. The area is very narrow. 



The figures 1 a-g, Plate xxi, are of this species from the Marcellus shale ; the profile 

 view is from a specimen which is much more convex than usual. 



When destitute of the cardinal spines, this species resembles the young of 

 Tropidolepius carinatus; but is readily distinguished when the cardinal spines are 

 present, and their direction is a characteristic feature of the species. 



The shell described as Chonetes laticosta occurs in the Corniferous limestone 

 and in the Hamilton group : the form is similar to C. mucronata of the Marcellus 

 shale. The ventral valve is often quite gibbous, regularly rounded in the middle, 

 suddenly depressed towards the cardinal angles, which are flattened and rarely 

 extended beyond the width of the shell. The dorsal valve is moderately concave, 

 sometimes very gently concave in the upper part of the valve and more abruptly 

 curved towards the front. 



The surface is marked by from twelve to twenty usually simple rounded or 

 Bubangular stria;, a few of which bifurcate towards the margin, or are increased 

 by intercalation of one, two or three others towards the margin, and have a 

 greater or less space between them. This gives considerable variety of aspect to 

 the surface ; and in all the individuals in the limestone, the striae are fewer and 

 stronger than in those from the Marcellus shale. The specimens from the Hamil- 

 ton group more nearly resemble those from the limestone. 



The cardinal margin is marked by two or three spines on each side of the 

 apex, with rarely indications of a fourth spine nearer the apex. These spines are 

 bent abruptly downwards, and directed outwards almost parallel to the hinge- 

 line, and in some instances the outer one appears to be a continuation of the 

 cardinal extremity (see figures 1 a-c, Plate 20; and figure 1 a-g, Plate 21). 



The area of the ventral valve is narrow and linear, and the dental lamellae 

 are prominent. In the interior there is a strong median angular ridge or septum 

 reaching more than half the length of the valve. The dorsal area is scarcely more 

 than the thickness of the shell. The cardinal process is narrow and scarcely bifid 

 at the extremity, and is supported on each side at its base by an oblique ridge, 

 and continued in a median longitudinal ridge. 



