﻿98 



BRITISH CARBONIFEROUS BRACHIOPODA. 



Spec. Char. Shell very variable in shape, transversely ovate or oblato-deltoidal ; wider 

 than long ; dorsal valve gibbous, most elevated near the front, evenly convex at the umbone j 

 mesial fold large, and more or less prominent. Ventral valve less convex than the opposite 

 one, with a sinus of moderate depth, commencing at a short distance from the beak, and 

 extending to the front. Beak small, much incurved, and contiguous to the umbone ; 

 foramen minute, placed under the extremity of the beak, and but rarely visible in full- 

 grown shells. Each valve is ornamented with from nine to fourteen ribs, which become 

 obsolete as they approach the beak and umbone ; from three to six occupy the fold and 

 sinus. 



Three examples measured : 



Length 12, width 17, depth 12 lines. (Martin's type.) 

 „ 24, „ 24, „ 20 „ 



06s. This, and R. acuminata, are certainly the Rhynchonellce at present known in the 

 Carboniferous period which appear to have attained the largest proportions, and although 

 believed by the generality of palaeontologists to be specifically different, and in general 

 easily distinguishable, it cannot be denied that there are exceptional examples, which from 

 their intermediate shape would go some length to support the opinion expressed by 

 Professor M'Coy, viz., that R. pugnus might be nothing more tha^ a variety of Rh. 

 acuminata} 



In 1809 Martin published two somewhat enlarged representations of what we must 

 consider to be an abnormal shape and injured specimen of the species under description ; 

 nor did Sowerby neglect to mention that had he not possessed the individual specimen 

 (which I have also seen and drawn) he would have felt some uncertainty as to its 

 identity with the specimens he subsequently figured in tab. ccccxcvii of the 'Mineral 

 Conchology.' 



In very large, full-grown examples, such as in figs. 3 — 6 of my plate, the posterior half 

 or upwards of each valve is evenly convex, and exhibits no trace of ribs, but these are 

 always present round the margin and on the anterior portion of the valves. This is also 

 the case with the generality of young shells ; but in the larger number of middle-sized 

 specimens the mesial ribs, or those which compose the fold and sinus, extend to more than 



1 1 British Palaeozoic Fossils,' p. 381. Professor M'Coy proposes to subdivide his Hemithyris acuminata 

 into four named varieties, viz. : 



" 1st var., — acuminata (Martin). Front sinus very high, acutely angular, few or no traces of mesial 

 plaits, nor lateral marginal plaits, except in very large specimens. 



" 2d var., — platyloba (Sow.). Transversely ovate, plaits obtuse. 



" 3d var., — pugnus (Martin). Rhomboidal tumid ; three to six mesial, and three or no lateral, short, 

 strong plaits. 



" 4th var., — mesogonia (Phill.). Form and other characters exactly as in the type, var. acuminata, 

 but the width less than one inch." 



