Terebratula undulata 

 Spikifer undulatus 



(?) — HYSTERICUS (?) 



132 PERMIAN FOSSILS. 



Spirifer undulatus, J. de C. Sow. Phillips, Encyc. Met., vol. iv, p. 617, 1834. 



Von Buch, Ueber Delthyris, p. 37, 1834. 

 Thomson, Outlines of Geology, &c., vol. ii, p. 326, 



1836. 

 Deshayea, Lamarck's Animaux sans Vertfebres, 



2d ed., vol. vii, p. 371, 1836. 

 Le Cocq., Transl. of Von Buch, Mem. Soc. Geol. 



de France, t. iv, p. 183, pi. vii, fig. 6. 

 Morris, Catalogue, p. 131, 1843. 

 Brown, Fossil Conchology, p. 110, pi. 50, figs. 



27,28, 1843. 

 Lyell, Elements of Geology, p. 416, fig. 241, 1838. 

 De Verneuil, Bull, Soc. Geol. de France, 2""' serie, 



vol.i, p. 28, 1844. 

 Geol. Russ, vol. i, p. 222, 1845. 

 Idem, vol. ii, p. 173, pi. vi, fig. 12, 1845. 

 (?) — (EsPECE indetermin^e), De Fern. Loo. cit., fig. 13. 



— UNDULATUS, /. de C. Sow. Tennant, Strat. List., p. 89, 1847. 



— ALATUS, Schl. King, Catalogue, p. 7, 1848. 



— UNDULATUS, J. de C. Sow. Howse, Trans. T. N. F. C, vol. i, p. 254, 1848. 



Diagnosis. — "Transversely elongated, very convex, with pointed extremities, radiated 

 transversely, and deeply striated ; front elevated, with a rounded sinus ; radii obtuse, 

 about 16 on each side the central elevation ; beaks a little removed ; area flat, 

 narrow." (J. de C. Sow.)^ 



This species has often been confounded with Trigonotreta alata, both of which 

 simulate each other; but T. undulata differs from T. alata in being more tumid at 

 the umbones, more produced in front, and a narrower shell : the last difference is 

 well displayed in young specimens of both species, as represented in figs. 10 and 16, 

 PI. IX.^ Both species agree in other respects. 



Mr. J. de C. Sowerby having kindly favoured me with an examination of the 

 original specimen of this species, figured in the ' Mineral Conchology,' I embrace the 

 present opportunity of drawing up from the same the following description, which, 

 with Mr. Sowerby's diagnosis, and the figures herein given (vide PI. IX, figs. 1, 2, 3), 

 will enable the reader to form a tolerably correct idea of its characters. Semi- 

 elliptical in marginal outhne : bi-areagerous : very inequivalved : two inches in width, 

 and an inch and an eighth in length, that is, from the umbone to the termination of the 

 sinus-prolongation in the large valve. Large valve moderately convex : median sinus 

 broad ; rather deep ; with a faint median rib : lateral surfaces each with fourteen or 

 more cord-like ribs, about half of which have resulted from dichotomy on the umbonal 

 region : umbone much incurved, and projecting considerably behind the hinge line : 



^ Mineral Conchology, vol. vi, p. 119. 



2 Since the plates were engraved, I have collected young specimens half the size of those in the figures 

 referred to, showing the same differences. 



