276 APPENDIX TO THE 



beak beyond and above the cardinal edge, in C. comoides, as good specific distinctions 

 whereby to separate it from the Llangollen shell, which possesses a very large, rounded 

 beak and narrow area. I am not, however, quite so certain as to the absolute value of 

 these characters in the forms under discussion ; but as my distinguished friend so strongly 

 advocates their separation, I will provisionally adhere to his view, and retain the term 

 comoides for Sowerby's type, and that of Llangollenm for those represented in PI. XLV, 

 figs. 1 — 6 ; and PI. LV, figs. 9, 10. They will be provisionally characterised in the 

 following manner. 



Productus comoides, Sow. PL XLV, fig. 7; PL XL VI, fig. 1 ; and PL LV, figs. 6, 7, 8. 



Sp. Char. Shell large, transversely semicircular, concavo-convex ; hinge-line straight, 

 nearly as long as the width of the shell; ventral valve convex, very thick and wide; beak 

 not protruding, or but very slightly so, beyond the level of the cardinal edge ; fissure 

 triangular and wide, partly arched over by a small pseudo-deltidium. Dorsal valve 

 moderately concave, much thinner than the opposite one ; area narrower than that of 

 the ventral valve. Externally, both valves are covered with exceedingly fine and contiguous 

 longitudinal striae (four or five occupying the breadth of a single line). In the interior of 

 the ventral valve (the only one known) and under the extremity of the beak there 

 exists a pyriform muscular depression, which extends to about half the length of the valve 

 (PL LV, fig. 8). This cavity is longitudinally divided into three almost equal portions; 

 the central division contains, on a wide, flattened elevation, two pair of muscular impressions, 

 situated one above the other. Those nearest to the extremity of the beak are due to the 

 adductor (a), while the smaller, circular or oval pair (c) are supposed to have afforded another 

 point of attachment to the same muscle. On either side of this central elevation are 

 situated larger and deeper scars, which are due to the cardinal or divaricator muscle. 



Dimensions variable ; the largest specimen I have seen measured 3^ inches in 

 length by rather more than 6 inches in breadth, while Sowerby's typical specimen, 

 when entire, must have measured 3 inches in length by somewhat less than 4£ inches in 

 width. 



The grounds for locating this species in the sub-genus Chonefes have not been clearly 

 made out. I will therefore leave it provisionally with Productus, as was originally done 

 by Sowerby. 



Sowerby states that his specimens (from which my description was taken) were from 

 the wayboards between the limestone under the coal at Llangaveni, in Anglesea. 



