112 PERMIAN FOSSILS. 



It is quite unnecessary at the present day to enter into any details proving the 

 distinction between HypotJiyris and Terebratula. In addition to what I have elsewhere 

 published/ the subject has been ably elucidated by Mr. Morris ;^ and no conchologist, 

 I believe, has demurred to the separation. 



Hypothyris appears to have withstood all the changes which organic nature has 

 been subject to on our planet ; as species have lived and succeeded each other from 

 almost the earliest organic period to the present moment. They were most abundant 

 during the primary and secondary periods ; few are known to have existed during the 

 tertiary; and only two species appear to be living at the present time. The one generally 

 known, Hypothyris psitiacea^ has an extensive geographical range in the northern 

 hemisphere, having been found in the icy seas of the arctic circle, and on the coast of 

 Northumberland.* 



The next genus is now for the first time proposed for a singular group of shells 

 which have long been without a proper standing place in our conchological systems. 



Genus Isorhynchus^ King. 



Type TerebratuUtes cequirostris, Schlotheim, as represented by De Vemeuil in the 

 ' Geologic de la Russie d'Europe,' vol. ii, pi. iii, fig. 1 . 



Biaynosis. — Sub-globular ; sub-sequivalved ; tumid behind, and compressed in 

 front ; slightly areated ; generally striated ; and with large punctures. Umbdnes of nearly 

 equal size ; that of the large valve foraminated at its apex. Large valve with two long 



de France' has made typical of the former genus ; but this I can say, the name Rhynconella cannot super- 

 sede that of Hypothyris for a genus typified by the Atrypa cuboides, — for this reason, that such a genus 

 (without going back to the year in which it was provisionally proposed by Professor Phillips, which i§ not 

 allowable in the present case) was defined both by Mr. Morris and myself, and also typified by one of us, 

 two years previously to M. A. d'Orbigny's resuscitating the Waldheimian name Rhynconella. In making 

 these observations I wish it to be understood, that I shall most cordially agree with what M. A. d'Orbigny has 

 done connected with Rhynconella, provided it does not clash with Hypothyris : it is very possible this is the 

 case, considering that this author has been describing cretaceous HypothyridcB, which have certainly some 

 appearance of being generically distinct from the normal species of the Palaeozoic rocks ; — but I am not aware 

 of the distinction having yet been established. 



1 Vide Ann. and Mag. of Nat. Hist., vol. xviii, pp. 32-6. 



2 Quarterly Journal of the Geological Society, vol. ii, part i, pp. 382-9. 



^ The other species, named by Mr. G. B. Sowerby, Terebratula nigricans, and described in his beautiful 

 ' Thesaurus Conchy liorum,' has not yet had its locality determined. 



* Vide Ann. and Mag. of Nat. Hist., vol. xviii, p. 238; and Forbes and Hanley's 'British Mollusca,' 

 vol. ii, p. 348. The specimens which I procured on the coast of Northumberland were an entire shell and a 

 rostral valve ; the former, which is the original of the figures in the ' British Mollusca' (pi. Ivii, figs. 1, 2, 3), 

 is now in Mr. J. G. Jefirey's rich collection of British shells, and the latter I presented to the Museum of 

 the Natural History Society of Northumberland, Durham, and Newcastle-on-Tyne. 



5 Etym. iaot, equal; pvyxos, a beak — in allusion to the nearly equal size of the umbones. 



