﻿RHYNCHONELLA. 



91 



elevated or gibbous ; profile much arched, especially at the umbone, which projects further 

 than the extremity of the beak ; externally the valve is divided into three portions or 

 lobes, of which the central one forms a mesial fold of but small elevation, and composed 

 of three, four, or five ribs, these becoming obsolete close to the umbone; the lateral 

 portions or lobes are inflated and smooth. In the ventral valve the beak is small and 

 much adpressed, the sinus deep and wide, with two or three longitudinal ribs along the 

 middle ; the lateral portions of the valve are smooth, and more or less inflated, usually 

 extending lower down than the margin, and to meet which they are abruptly bent. The 

 entire external surface of shell is covered Avith minute, longitudinal, radiating striae. 

 Dimensions and proportions very variable; three specimens have measured — 

 Length 14, width 18, depth 15 lines. 



3 > 12, ,, 14, ,, 13 „ 

 Obs. Some difference in opinion has been expressed as to the specific value of 

 Terebratula reniformis j 1 thus, in 1843, it was placed by Professor de Koninck among the 

 synonyms of B. acuminata, but from which the same author removed it at a subsequent 

 period. 



In the same year (1843) Sowerby's T. cordiformis had also been considered by Pro- 

 fessor de Koninck as a variety of B. acuminata, and my distinguished friend still adheres 

 to the same opinion, although the generality of palaeontologists maintain the three as 

 separate species. How far this last view is correct I am not yet perfectly prepared to 

 decide, notwithstanding the long and careful examination I made of the original types, 

 along with many other specimens of the same species. 



It is very difficult, nay, quite impossible, to express by words alone those minute 

 differences which at times distinguish two closely allied species ; I have, therefore, spared 

 no trouble in carefully selecting and representing all those specimens which may enable 

 the reader to form an opinion for himself, and especially so as I have still some misgivings 

 as to the exact position B. cordiformis should occupy ; that is to say, whether it should be 

 viewed as a distinct species, or as a variety of B. reniformis or B. acuminata. 



The general characters and facies of Bh. acuminata and B. reniformis appear to me 

 sufficiently defined, and not to be confounded > the remarkable inflation of the umbone of 



1 I here reproduce the original description : 

 "Terebratula reniformis, Sow. Vol. v, p. 496, tah. ccccxcvi, figs. 1 — 4. 



"Spec. Char. Reniform ; middle furnished with three or four longitudinal, rounded ridges, terminated 

 by acute plaits in the much-elevated margin in the front ; sides inflated below the entire edges; a much- 

 rounded, almost two-lobed, inflated, and more or less depressed, shell. The form of the ridges and inter- 

 mediate furrow is very remarkable, being rounded, while the corresponding notches in the margin are 

 acute-angular. The peculiar form of the sides of the beaked valve, which are inflated so as to hang below 

 the edges, will distinguish all the varieties of this species. Very abundant in the mountain limestone of 

 Dublin and Cork." 



