220 



THE GEOLOGIST. 



somewhat longer than the greatest width of the shell, with rectangular or 

 slightly acute and extended terminations. Both valves are provided with 

 narrow sub-parallel areas, the ventral one, which is the largest, being divided 

 by a small fissure, partially covered with a _ pseudo-deltidium ; while in the 

 middle of the ventral one there exists a prominent Y-shaped cardinal process. 

 The ventral valve is moderately convex, and flattened towards its auriculate 

 cardinal extremities. The beak, which is small and incurved, does not overlie 

 the hinge-line ; while the dorsal valve assumes in different specimens a greater 

 or lesser degree of concavity, and follows the curves of the opposite one. 

 Exteriorly the surface of the ventral valve is covered with numerous small 

 thread-like radiating striae, which increase in number by occasional bifurcation, 

 or interstriations at various distances from the beak, so that as many as one 

 hundred and twenty ribs may be counted round the margin of certain specimens, 

 while at irregular distances small spines projected from the rounded surface of 

 the striae. In addition to these, on each side of the beak there exists along the 

 cardinal edge from five to nine slanting tubular spines, which become longer 

 and larger as they approach the extremities of the cardinal edge. The surface 

 of the dorsal valve is striated as in the ventral one ; and minute perforations 

 or punctures may be perceived over the entire surface of the shell, and which 

 are the exterior orifices of the canals which traverse the shell, as in Productus. 



In the interior of the ventral valve there exists a tooth on each side of the 

 small fissure, and which fitted corresponding sockets in the opposite valve, 

 while the occlusor and divaricator muscular impressions are very similar to 

 those of 'Productus giganteus, but proportionately much smaller, as may be 

 seen by a glance at the figures of the two species. Under the cardinal pro- 

 cess, in the interior of the dorsal valve, a mesial ridge or plate extends to 

 nearly two-thirds of the length of the valve, and on either side may be observed 

 two weU defined occlusor muscular scars, the four being comparatively lai'ger 

 or more spread out than is generally the case with Productus ; while outside, 

 and in front of these, are situated the reniform impressions, 



Chonetes Hardrensis is a small species, rarely attaining in Scotland eight lines 

 in length by twelve in width. It varies, likewise, in the number of its striae, 

 and these are very much finer or coarser in some specimens than in others. 



A small variety, which I take to be the same species (pi. ii., fig. 7), occurs 

 by millions in certain localities, such as at South Hill, Campsie, in a bed of 

 shale on the horizon of the Hosie limestone of the Carluke section ; and are 

 associated with Sp. TJrii in almost equal abundance. The striation in this small 

 Chonetes is generally so fine that it can hardly be distinguished without the 

 help of a lens ; and although it has been thought that this little form might 

 constitute a distinct species, I am still inclined to view it simply as a small 

 variety of C. Hardrensis. 



The determination and study of the present species has given me much 

 trouble ; and although I have spent much time in the endeavour to arrive at a 

 satisfactory conclusion, it is not without some hesitation that the term Har- 

 drensis \^ here provisionally retained; provisionally, because I am at present 

 unable to determine whether Phillips' Devonian shell is the same as that to 

 M'liicli Schlotheim in 1820 applied the denomination Sarei/iidata, as Prof, de 

 Koninck's illustrations of this last differ so much from those given by Prof. 

 Sclniiir and some otlu^v pahvoutologists, I am likewise uncertain whether 

 J. de C. Smvorby's J.c])/. sard Ida (ISlO) be really a synonym of the last-named 

 slicll, or diHVrcni IVoin PJiillips' Hardrensis, as "has been stated to be the case 

 by some authors ; aiul lastly, because my learned friend, Prof, de Koninck, 

 who has paid so much attention to the species of the genus, maintains a dif- 

 l(MTut o]nnion to that here recorded, but not absolutely "denying the possibility 

 of mine being correct. 



