1853.] DAVIDSON ON THE CHONETES COMOIDES. 205 



None of the examples I have been able to examine were provided 

 with cardinal spines, nor do the figures published either by Sowerby 

 or Count Keyserling exhibit any ; but, from various indications along 

 the cardinal edge of one or two of Mr. Ormerod's specimens, it seems 

 probable that they originally existed, and were (as observed by 

 M. de Koninck) numerous, short, and slender. 



In dimensions, as well as in external shape, this species varies to 

 a considerable extent, — a circumstance common to almost every 

 species composing the class. Thus some examples are of a broad 

 semicircular form, with their greatest width at or near the hinge-line ; 

 while others are almost circular, the greatest width being found 

 towards the middle of the shell*. Thus two extreme individuals 

 have presented the following measurements : — 



Length 3 inches. Width 6 inches. 



Ditto 2 inches 9 lines. Ditto 3 do. 



The Chonetes comoides does not appear to be a very common 

 species, and occurs only in the Mountain-limestone. The following 

 British localities have been recorded. Llangaveni in Anglesea, by 

 Sowerby; Llanymynech, by Mr. Yates; Tidenham Chase (Glou- 

 cestershire), near Chepstow, Treflach Wood, S.W. of Oswestry, and 

 Beaumaris, Anglesea, by Mr. G. W. Ormerod ; Bundoran, County 

 Donegal ; and Lough Erne, Fermanagh, &c. 



* Since communicating the above observations to the Geological Society, Mr. D. 

 Sharpe has obtained the loan of the original specimens of C comoides, figured in 

 the ' Min. Con.,' and, having removed the matrix which encumbered some portions 

 of the area, has found that fig. 1 in Plate VIII. exactly agrees with the typical speci- 

 inev, but that, in his opinion, the other examples illustrated in Plate VIII. figs. 2 to 

 8, may, perhaps, belong to another species, as they seem externally more globose, 

 and have a smaller area, on which he was unable to discover the diagonal lines ob- 

 servable in the types oi Chonetes comoides; Mr. Sharpe considers also that in the 

 interior of the ventral valve there is a perceptible difference in the shape of the 

 muscular impressions as well as in the development of the mesial septum. But, 

 having since had the opportunity of examining along with Mr. Sharpe and Mr. 

 Salter tlie specimens above alluded to, I did not feel convinced that sufficient 

 grounds existed for the establishment of two distinct species (an opinion in which 



1 am supported both by Mr. Salter and Mr. Woodward). The original type of C. 

 comoides does not appear to me to dirFer materially in its convexity from several of 

 those figured in my Plate. The area, I am ready to admit, is certainly wider in 

 both of Mr. Sowerby's specimens, than in those belonging to Mr. Ormerod (figs. 



2 to 8). Mr. Sowerby's second example (not figured in the ' Min. Con.') is an 

 incomplete interior of the ventral valve, in which from the shell being both young 

 and shallow, the muscular impressions could not be as deep or as indented as in the 

 adult, very convex, and thickened valves, such as the original specimen figured in the 

 ' Min. Con.,' or those illustrated in my Plate, figs. 6 to 8 ; nor could I perceive that 

 the interior of Mr. Sowerby's specimen varied in any essential particular from those 

 more perfect specimens I had examined, and all I feel at present disposed to admit 

 is, that those more circular shapes with a narrow area may be varieties of the 

 C coiHoides. No difference in external striation appears to exist, nor are all the 

 examples circular, which is proved by my figs. 2 and 6. I need hardly recall to the 

 reader's memory how much some individuals of one species vary in shape, some 

 being wide, others elongated, depressed, or gibbose, with a large or narrow area, — 

 circumstances often due to habitat or other physical conditions, forming innumei able 

 varieties of a single species; but, since a difference of opinion to that advocated in 

 this paper has been expressed, the subject may be considered unsettled and worthy 

 of the notice of palffiontoiogists. — [Jan. 6, 1854.] 



p2 



