138 



BRITISH CARBONIFEROUS BRACHIOPODA. 



the shell did not possess a sufficient thickness, as they never influenced the regular curve 

 or convexity of the exterior of the valve. In all the Productid^e we therefore find muscles 

 destined to open and close the valves, as well as evidence that they possessed spiral arms, 

 and which have been justly, I think, supposed to be brachial appendages, which subserved 

 at once the function of gills and of sustentation. 



In C/toneles, the muscular impressions are somewhat similar to those of Productus, but 

 of more often comparatively smaller proportions, as may be perceived by a glance at the 

 figures I have given of both. 



The determination and arrangement of the British Carboniferous species of Productus and 

 Chonetes has demanded a lengthened examination, for much confusion still existed among 

 the synonyms, notwithstanding the able and valuable researches of Professor de Koninck. 

 Through the kindness of several friends, I have been enabled to study the original types of 

 the larger number of species, as well as a very extensive series of specimens ; and although 

 I have rejected a great number of so-termed species, and been able to add several others 

 new to Great Britian, 1 should still urge further research upon these very interesting shells, 

 as it has not been always possible to determine the specific claims of some among them 

 with sufficient accuracy. 



After much examination, and in some cases much uncertainty, the following twenty- 

 eight species have been retained, and which may be arranged in the following three 

 groups. 1 



1. Striati, or Semireticulaii 



1. 



Productus striatus, Fischer. 



2. 



— 



giganteus, Martin. 



var. hemispl«ericus, Sowerby 



3. 



— 



latissimus, J. Sowerby. 



4. 



— 



Cora, D'Orbigny. 



5. 



— 



semireticulatus, Martin. 



G. 



— 



costatus, Sowerby. 



var. 1 muricatus, Phillips. 



/ • 



— 



longispinus, Sowerby. 



8. 



— 



sinuatus, De Koninck. 



9. 



— 



humerosus, Sowerby. 



10. 



— 



Margaritaceus, Phillips. 



11. 



— 



undatus, Defrance. 



12. 



— 



arcuarius, De Koninck. 



13. 



— 



carbonarius, De Koninck. 



14. 



— 



ennineus, De Koninck. 



15. 



— 



proboscideus, DeVerneuil. 



16. 



■ — 



Wrightii, Davidson. 



17. 



— 



tessellatus, De Koninck. 



1 Those groups and species most nearly related have been connected by the means of a brace. It is 

 difficult, with words, to explain the exact contour, or shape, of such variable shells as the Producti ; but 



