RHYNCHONELLA. 79 



Diagnosis. Shell variable, irregularly triangular, wider than long; beak acute, 

 more or less produced and recurved; foramen small, entirely surrounded or almost so by 

 the deltidial plates, a very small portion being completed by the umbo, the beak is 

 sometimes so much recurved, as hardly to allow any space for the passage of the pedicle 

 muscular fibres ; beak ridges well defined, leaving a flat or concave false area between 

 them and the hinge margin, and not much indenting the smaller valve, which presents 

 an elevated regular convex curve from the umbo to the front, ornamented by a variable 

 number of costse, two, three, or four in different examples, forming a central elevated 

 mesial fold, corresponding with one, two, or three plaits situated in the sinus of larger 

 valve ; on either side of this mesial fold and sinus on both valves are seen three or 

 four lateral costse, not varying much in number on different specimens, so that the 

 smaller valve may be ornamented by eight, nine, or ten plaits ; the number on the 

 mesial fold being independent of those on the sides ; the costae in general becoming 

 visible, and produced only at a little distance from the beak and umbo. Structure un- 

 punctuated; length 10, width 12, depth 9 lines. 



Obs. This shell has caused great confusion from its variable shape, no doubt 

 due to local causes ; and it is not without many comparisons and researches that I 

 made up my mind to adopt M. D'Orbigny's views, viz., to consider Professor Phillips's 

 Ter. triplicate/, and bidens as synonyms of Schlotheim's T. variabilis, which species was 

 unfortunately established on a very poor and exceptionable specimen, so that his figure 

 in no way represents the general and well-developed types of the species, so much so, 

 that from not being able to make up my mind to their identity I was led to 

 figure both separately in different plates, but having since re-examined a multitude of 

 examples, and been unable to discover permanent distinguishing characters, 1 am obliged 

 to admit Schlotheim's priority. It will not, however, be useless to mention some points 

 connected with the history of this species ; Von Buch, in his important work on 

 ' Brachiopoda,' admits both T. variabilis and triplicata, stating at the same time, that the 

 difference between them is very small. Professor Phillips's Ter. triplicata and bidens 

 belong to the same form, and not to the B. cynocephala of Richard, as we have already 

 explained under the description of that species, although both are found in the same 

 neighbourhood, but not in the same beds. B. variabilis, or triplicata and bidens, being 

 only varieties with two or three plaits on the mesial fold, and it is always objectionable to 

 name species from the number of costse ornamenting their surface, as these constantly 

 vary to a greater or less degree on different specimens, B. variabilis presenting at times 

 from two to five plaits on the mesial fold, while the types of Schlotheim and Phillips 

 have but two or three ; most writers seem to have retained both these authors' 

 names, while some, as Von Buch, M. Deslongchamps, Professor Bronn, Rouillier, 1 

 &c, have considered bidens and triplicata as the same type ; but the last two authors 



1 The references are given in the synonyms placed at the head of this description. 



M 



