204 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 14, 



rior of the ventral or dental valve we are at present in a condition to 

 offer a detailed description and exact illustrations*. 



Under the extremity of the more or less produced and incurved 

 beak, and between the projecting teeth, is seen a depression of mode- 

 rate depth, which was almost entirely occupied by the cardinal pro- 

 cess of the dorsal valve. At the base of the fissure above described, 

 originates a lai'ge deep pyriform muscular 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 are situated four elongated muscular impressions ; 

 and these are separated to a greater or lesser extent by three longitu- 

 dinal 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 muscles ; but no traces of 

 peduncular muscular impressions could be traced on any of the spe- 

 cimens placed at my disposal for examination, nor are the muscular 

 scars equally deep in all examples ; the separathig ridges are often 

 but faintly developed, especially in those cases in which the valve is 

 shallow. 



Sowerby stated that the Chonetes comoides is " very thick and 

 rough within ; " the same was noticed by M. de Koninck and others ; 

 but these authors do not seem to have observed the very great 

 disproportion in the relative thickness of the valves so beautifully 

 exemplified in the fragment (PI. VIII. fig. 5)f . The considerable 

 weight of the ventral valve precludes all possibility of the animal 

 having lived attached by its short and delicate cardinal spines J, as 

 some have imagined, and renders more probable the idea expressed 

 by M. d'Orbigny (when describing Productus), that the animal 

 lived on muddy bottoms with its smaller {dorsal) valve uppermost, 

 because no aperture exists large enough to have afforded passage to 

 muscular fibres of sufficient strength to have supported so pon- 

 derous a shell, nor could the physical power of the animal have 

 sufficed to have opened its valves, had the weighty and often gibbose 

 ventral valve been the upper one ; but, on the contrary, by supposing 

 this to have been the lower one, not much exertion would have been re- 

 quired to slightly raise its dorsal valve, which was comparatively thin. 



* Some months ago, while examining the carboniferous Brachiopoda, preserved 

 in the Museum of the Geological Society, my attention rested upon two imperfect 

 but remarkably massive circular valves of a Brachiopod labelled '' Productus 1," 

 and stated to have been discovered at Llanymynech by Mr. J. Yates. These valves 

 differed so materially from all those with which I was then acquainted, that 1 was 

 tempted to refer them, with doubt, to some species of Orlhis bearing outward re- 

 semblance to the U. resupiiiata (^see a note, p. 103 of my 'Introduction ') ; but 

 shortly after, having received from Mr. G. W. Ormerod the loan of several perfect 

 examples similar to those in the Society's Collection, I at once discovered my 

 mistake, since they belong to a variety of the C. comoides of Sowerby, an opinion 

 previously expressed by Mr. Salter on inspecting Mr. Ormerod's specimens at 

 Manchester. 



f 1 do not quite understand Count Keyserling's section of Chonetes, as jio free 

 space seems to exist for the animal, 



X Their presence in this species has not as yet been satisfactorily demonstrated. 



