﻿102 



BRITISH CARBONIFEROUS BRACHIOPODA. 



valves more or less convex, sometimes very gibbous, beak moderately produced, incurved, 

 and exhibiting a small circular foramen under its angular extremity, which is surrounded 

 and slightly separated from the hinge-line by a deltidium ; medial fold large, almost 

 square, and most elevated close to the front, when it is suddenly deflected, so as to meet 

 the corresponding margin of the opposite valve; sinus in ventral valve of moderate 

 depth ; ribs numerous and angular, extending over the whole surface, and numbering from 

 ten to about twenty-four in each valve, of which three to nine compose the fold, and two 

 to eight the sinus ; but five on the fold and four in the sinus is the general number. The 

 plaits that cover the lateral portions of the dorsal valve are very much curved, while those 

 of the ventral one are nearly straight, with their extremities bent upwards. The ribs are 

 longitudinally grooved along their median portion to some distance from the margin. 

 Dimensions very variable ; three examples have measured : 

 Length 10, width 14, depth 11 lines. 

 33 9, ,, 13, ,, 10 „ 



33 ,, 11, „ 5 ,, 



Obs. This Rhynchonella is very abundant in the Carboniferous rocks, but varies so 

 much in shape, according to age and specimen, that several so termed species have evidently 

 been manufactured out of what I conceive to be mere differences in shape, race, and even 

 malformation of a single species. 



B. pleurodon does not appear to have ever attained very large proportions, and is 

 distinguishable from B. pugnus, B. acuminata, and B. reniformis, by the angular ribs 

 which cover the entire surface of its valves. When young, with dimensions of from two, 

 three, and sometimes more lines in length, both valves are at times much compressed, the 

 fold and sinus being but little elevated or depressed beyond the regular convexity of the 

 valves, a small longitudinal depression extending likewise along the median portion of 

 the umbone of the dorsal valve. In the fry the shell is at times somewhat triangular, the 

 width being equal to the length, but with growth the shape becomes more transverse, and 

 rapidly increases in depth or convexity, the fold and sinus assuming likewise their charac- 

 teristic appearance. Many young specimens of undoubted B. pleurodon possess but three 

 ribs on the mesial fold, and two in the sinus, but in the larger number of full-grown 

 shells five were prevalent, although we sometimes pick up specimens with as many as 

 from six to nine ribs on the fold ; but these are exceptions, and of much less common 

 occurrence. 



It may now be desirable to examine what are those so-termed species which I have 

 been tempted to consider as simple variations in shape or synonyms of B. pleurodon. 



First — Bh. {Terebratula) Mantice, Sowerby, pi. xxiii, figs. 15, 15 a, b, c (' Min. Con./ 

 tab. cclxxvii, fig. 1, May, 1821). 



Sowerby founded his T. Mantice from the inspection of a single specimen, said to 

 have been collected in Ireland by a Mrs. Mant, but unfortunately the figure published 

 in the 'Mineral Conchology' does not convey an accurate idea of the shell, and I 



