TEREBRATULA. 15 



nation. The frontal margin of the ventral valve, in the greater number of .well-shaped 

 and adult individuals of T. sacculus, presents a convex curve, indentiug to a lesser or 

 greater degree that of the opposite valve, but without producing in it any sensible mesial 

 elevation. Many examples, wherein the sinus presents a gradual concave curve, bear 

 much resemblance to some young conditions of the Permian T. elongata and its var. 

 sufflata, so much so that it would be difficult to verbally describe the minute differences 

 which seem to distinguish both species, but which, when full grown, are sufficiently 

 evident. 1 From T. vesicularis, De Koninck, T. sacculus is very often still more difficultly 

 distinguished ; and although both shells are pronounced distinct by Professors De Koninck 

 and M'Coy, I have not been able to arrive at so decided an opinion, for although the 

 frontal wave in most examples of Martin's shell is formed by a single curve, there appears 

 to exist a very gradual passage leading to the triundate wave and frontal plication 

 observable in well-characterised examples of T. vesicularis. 



As the above-named authors seem to differ with me in this particular, I have pro- 

 visionally described both under a separate denomination. Some naturalists have proposed 

 to consider T. pentaedra, Phillips, and T. didyma, of Dahnan, as synonyms of Martin's 

 T. sacculus, but this mistaken notion has been already objected to by M. De Verneuil, at 

 p. 65 of vol. ii of the ' Geol. of Russia." I am likewise compelled to observe, that M. De 

 Koninck is mistaken while identifying Anomia lavis, in Ure's ' Hist, of Rutherglen,' 

 p. 313, pi. 16, fig. 9 (1793), with the species under description. It belongs to 

 Athyris ambiyua, Sowerby, a form that abounds at Lawrieston, as well as in the neigh- 

 bourhood of Carluke, whence Ure's examples were derived. 



Loc. T. sacculus is common in the mountain limestone. In England it is found at 

 Eyem, Middleton, Moneyash, Cronkstone, Matlock, and other Derbyshire localities ; it 

 occurs also at Bolland, Otterburn, Kendal, Malhara-moor, &c. In Scotland it is found 

 at Westlothian. Mr. Kelly mentions Ardagh, Millecent, and Little Ireland as Irish 

 localities. M. De Koninck has obtained it at Vise and Chokier in Belgium ; but, accord- 

 ing to M. De Verneuil and Count Keyserling, it would be rare in Russia, and has hitherto 

 been found but at Cosatchi-datchi, to the east of Miask, in the Oural, and at the mines of 

 Gerichof, Gouvernement of Tomsk, in Siberia. 



Terebratula vesicularis, Be Koninck. Plate I, figs. 25, 26, 28, 31, 32 ; Plate II, 



figs. 1 — 8. 

 Terebratula. vesicularis, De Koninck. Animaux Fossiles de la Belgique (Sup.), 



p. 666, pi. lvi, fig. 10, 1851. 

 Seminula seminula, M'Coy (not of Phillips). British Palaeozoic Fossils in the Camb. 

 Mus., p. 412, 1855. 



Spec. Char. Shell small, ovato-pentagonal, longer than wide, its greatest breadth 



1 Refer to what we have stated on this subject in Part IV, under T. elongata. 



