of Carhonifet'om Species. 215 



And the difficulty with which palseontologists have to contend 

 from what Prof. King calls simulating forms is, I think, over- 

 stated. It is said by this authority that many existing shells 

 have a tendency to simulate the distinctive features of other 

 allied species ; and, in support of this assertion, twelve British 

 shells are mentioned, which Prof. King is satisfied would have 

 been reduced to half the number had they occurred as palaeozoic 

 fossils. It may be observed, in the first place, that such shells 

 as simulate the distinctive characters of others can be but ques- 

 tionable species at the best; and the conchologist would not, 

 perhaps, be far wrong were he to anticipate the palaeontologist 

 by putting them together. This, in fact, has already been done 

 in respect to some of the shells that Prof. King quotes, by most 

 British naturalists. Astarte Danmoniensis and A. Scotica, for 

 instance, are usually considered to be identical ; and there are 

 few conchologists who admit Mya Uddevallensis to be distinct 

 from M. truncata — an opinion which Prof. King himself held 

 some time ago, as appears by his paper published in 'Ann. 

 Nat. Hist.' ser. 1. vol, xix. It is not to be denied, however, 

 that several of the shells mentioned are good species; but, as 

 most of them occur as Tertiary fossils, and have already been 

 identified as distinct species*, they would not appear to run 

 much risk of being confounded with others, even should they 

 ultimately attain an age as great in comparison as palaeozoic 

 fossils at present. 



EXPLANATION OF PLATE IV. 



Fig. 1. Cythere (Bairdia) Schaurothiana, Kirkby. Carboniferous speci- 

 men, left valve. Craigeuglen, Campsie. 

 Fig. 2. The same. Carboniferous specimen. Right valve. Craigenglen. 

 Fig. 11. The same. Carboniferous specimen. Lateral contour of left 



valve. Craigenglen. 

 Figs. 3, 4, 12. The same. Permian specimens, showing same aspects as 



before. Tunstall Hill. 

 Figs. 5, 6, 9. Cythere (Bairdia) plebeia, Reuss. Carboniferous specimens, 



showing left valve and lateral contour of same. Craigenglen, 



Campsie. 

 Figs. 7, 8, 10. The same. Permian specimens, showing same aspects as 



last. Tunstall Hill. 



[All the Entomostraca magnified 25 times.] 

 Fig. 13. Fenestella retiformis, Schloth. Permian. Non-celluliferous face ; 



magnified 6 times. Tunstall Hill. 



* Lutraria ohlonga and L. elliptica, for instance, occur together as 

 fossils in the newer Tertiary beds of Sussex (Quart. Journ. Geol. Soc. 

 vol. xiii. p. 61 ). In the same deposit Solen siliqua has been detected (ibid. 

 p. 63); and in another Tertiary bed Solen ensis occurs {ib. vol. xiv. p. 328). 

 Mya truncata and M. Uddevallensis also both occur in a fossil state 

 (Woodward's ' Mollusca,* p. 357, and Quart. Journ. Geol. Soc. vol. xiii. 

 p. 53). 



