of a feiv Brachiopoda. 253 



dissimilarities, while in some species of Rhynchonella, such as R. 

 lacunosa (PI. XIII. fig. 16), and others where the shell is very 

 thick, great differences are visible, but which are in reality only 

 specific, not generic. 



M. D'Orbigny was probably not acquainted with the interior of 

 this valve in his H. sub-TVilsoni, else he would have seen that 

 these impressions there differ much more from those of R. 

 psittacea than any he could point out between R. psittacea and 

 his Rhynchonella. 



From the above examination it is clear that no sound reasons 

 have been brought forward by the different authors who have 

 divided this type, to authorize the forming of separate genera ; 

 that the comparative study and minute inspection of the various 

 species has not been entered into, and that the characters assigned 

 as generic variations are inexact. 



Therefore the question now remaining is, what name these 

 shells ought to preserve : that of Rhynchonella is the oldest I 

 am acquainted with ; but if palseontologists can show good reason 

 why such forms as T. Wilsoni, sub-JVilsoni, cuboides, &c. are 

 not Rhynchonella, they cannot adopt that of Hemithiris (as justly 

 observed by Prof. King), Prof. Phillips having prior to M. D'Or- 

 bigny proposed the name of Hypothiris for such shells as R. cu- 

 boides. The genus Hemithiris, D'Orb., and Acanthothiris, D'Orb., 

 and all the other names above alluded to, must be placed among 

 the synonyms. Before concluding these few remarks, we men- 

 tion, that having been so fortunate as to discover in a quarry at 

 Nehou, interiors of both valves of R. sub- Wilsoni, D'Orb., and 

 internal casts of R. JVilsoni, Sow., in the Aymestry limestone 

 of Sedgiey, we have figured both in our plate, to show that there 

 exist good grounds for retaining these two specific names, since 

 the muscular impressions seem to vary in both. 



Plate XIII. fig. 8 to 11. R. sub-Wilsoni, D'Orbigny*. 

 Plate XIII. fig. 12 to 14. R. Wilsoni, Sow.f 



Riiyxchoxella Desloxgchampsii, Dav. 1852. PI. XIIL 

 fig. 15. 



Diagnosis. Shell inequivalve, irregularly lozenge-shaped, wider 

 than long, convex j smaller valve divided into three almost equal 

 lobes, the central one forming a greatly elevated mesial fold ex- 

 tending much beyond the lateral ones, which gradually slope to 



* The position and form of the apophvsarv lamella; were known to M. 

 Dcslongchamps many years back to be similar to those of R. decorata. 



t Several good internal casts of this shell exist in the Museum of the 

 Geol. Survey, which were placed at my disposal for examination by Prof. 

 Forbes and Mr. Salter. 



