CHONETES. 181 



always narrower in the smaller valve. In the middle of this las! there exists a 

 produced, trilobed, cardinal process, which enters and almost (ills a corresponding 

 triangular fissure in the ventral area, and which was probably partly covered or arched 

 over by a pscudo-deltidium. The external surface of the shell is finely striated. In the 

 interior of the greatly thickened ventral valve, and under the extremity of the beak, at 

 the base of the fissure between the projecting teeth, originates a large, deep, pyriform mus- 

 cular cavity, extending to beyond half the length of the valve, and occupying upwards of a 

 third of its inner surface, its greatest breadth being towards the centre of the shell. In 

 this depression arc situated four elongated muscular impressions, and these arc separated 

 to a greater or lesser extent by three longitudinal ridges, the central one of which is shorter 

 than the others, and assumes the character of a mesial septum ; the two smaller scars, 

 situated on either side near the central ridge, are due to the adductor, the outer and larger 

 ones to the cardinal or divaricator muscles, the muscular scars not being equally deep in 

 every example. Of the interior of the dorsal valve nothing is at present known. Dimen- 

 sions variable ; two British specimens have measured — 



Length 2 inches lh lines, width 3 inches 10 lines. 

 ,, 3 ,, o ,, ,, o ,, * 



Obs. Sowerby's specimens of this remarkable species are incomplete, and his descrip- 

 tions and illustrations consequently so, but he did not fail to mention that the shell is 

 " very thick and rough within," this allusion having reference to the ventral valve onfy" 

 for, as I have elsewhere shown, the convex valve was often four or five times as thick as the 

 concave or dorsal one. Sowerby's figures do not show the fissure which exists in the area 

 of the ventral valve, nor do they exhibit, the area of the dorsal one, which is so well dis- 

 played in a specimen preserved in the Bristol Institution Museum (PI. XLV, fig. 7). 

 No spines could be detected on any of the few examples that have come under m}' 

 observation, although some small circular holes could be perceived here and there, close to 

 the cardinal edge, in one of the specimens ; but it must likewise be remembered that the 

 cardinal spines were very small in some other species of Chonetes, and especially so in the 

 large C. papilionacea, and may, consequently, not have existed, or been destroyed, on the 

 few fossil examples we possess. 



It has been questioned whether the shell under description should be located with 

 Chonetes or with Prochfcti/x, and that we are not acquainted with the interior of the 

 smaller and most important valve ; still the area in both, and the strongly articulated 

 hinge, would be an anomaly in Proch'ctus, which, on the contrary, is the constant character 

 of Chonetes, and this alone would, at least provisionally, induce me to leave C. ? comoides 

 in the last-named sub-genus. It has also been doubted whether the shells, fig. 1 — 6 of my 

 PI. XLV, do really belong to Sowerby's species; and here, again, I must repeat what I 

 said in 1853, viz., " that I do not feel convinced that sufficient grounds exist for the 

 establishing of two distinct species (an opinion in which I was then supported by Messrs. 

 Salter and Woodward), the original type not appearing to us to differ materially in its 



