20 BRITISH TERTIARY BRACHIOPODA. 



Genua — Rhynchonella, Fischer. 1809. 



Shell more or less circular, elongated or transverse ; valves convex ; beak acute, slightly 

 or greatly recurved ; no true area ; peduncular perforation variable in form ; entirely or 

 partially surrounded by a deltideum, either lateral and rudimentary, complete, or tubular. 

 Surface of valves variously ornamented, rarely smooth, generally striated, plaited, or 

 costellated; structure non-punctuated, divisible into laminae of extreme tenuity; valves 

 articulating by means of two teeth in larger, and corresponding sockets in the imperforated 

 valve ; apophyses in smaller valve consisting of two short lamellae, separate and moderately 

 curved upwards, flattened and grooved, to which are attached the free, fleshy, spiral arms. 



Obs. We consider M. D'Orbigny's lately proposed genus Hemithiris as synonymous 

 with that established by Mr Fischer in 1809, under the name of Rhynchonella, both 

 genera being made up of shells of the same form, structure, apophysis, muscular impres- 

 sions, &c. ; the only distinction, according to M. D'Orbigny, consisting in the erroneous 

 statement, that the foramen in Hemithiris was deprived of deltideum ; while, in Rhyncho- 

 nella, it was complete and tubular. 



If we now examine with care some of M. D'Orbigny's types of Hemithiris, such as 

 H.psittacea and spinosa? we find that, in the first, the socket-walls do not form simply the 

 sides of foramen, but that there exist two narrow plates, gradually widening as they 

 proceed from the extremity of the beak and overlaying the socket walls, as we see in many 

 species, considered to be possessed of deltideum. In the second, II. spinosa, D'Orb., we 

 find regular lateral, deltideal plates, which do not completely surround the foramen, and 

 this becomes evident in specimens where the beak is not so much recurved as to conceal 

 the aperture. If, on the other hand, we cast our eyes on some of the shells admitted by 

 M. D'Orbigny as true Rhynchonellas, the deltideum is found disposed in the following 

 three ways : — 



1. Small and lateral, as in R. concinna, the foramen not entirely surrounded, a small 



portion being completed by the umbo. 



2. Surrounding the foramen, without tubular expansions, as in R. obsoleta. 



3. Surrounding the foramen, thickened and produced in the form of a tubular 



expansion, as in R. compressa, scaldinensis, vespertilio, &c. 



From the above it will be clearly seen that there is no important distinction between 

 the two genera; both, in our opinion, are referable to the genus Rhynchonella. These 

 views as to the form and value of the deltideum were published in a paper by 

 M. J. Deslongchamps, 2 and by Mr. Morris 3 some years back. The last-named author, 



1 M. D'Orbigny afterwards placed this shell into his genus Acanthothiris. 



2 Soc. Linneenne de Normandie, 1837. 



3 'Quarterly Journal of the Geol. Soc.,' vol. ii, Part I, pp. 382-9. 



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