UNDULATE. 59 



Ammonites of the formation in the collection made by Mr. Leckenby, now in the 

 Woodwardian Museum, Cambridge, including the specimens figured in the plates 

 accompanying his memoir on the Kelloway Rock of Yorkshire (' Quart. Jour, Geol. Soc.,' 

 1858, vol. xv) ; but no example of Trigonia jpaucicosta has occurred at that locality. 



Trigonia Painei, Lye. PI. XII, figs. 2, 3, 4, 5. 



Tkigonia GoLDFUSsii, Mom* and ij/ce^f. Monog. Moll. Gr. Ool., Pal. Soc, 1853, 



Bivalves, pi. v, figs. 18, 18 a; not Lyro- 

 don literatum, Goldfuss. 

 — — Morris. Catalogue, 1854, p. 228. 



Shell depressed, ovately trigonal; umbones antero-mesial, large, elevated, and 

 slightly recurved ; anterior border moderately produced, and elliptically curved with 

 the lower border ; hinge-border nearly straight, sloping obliquely, and terminating in an 

 oblique truncation of the posteal extremity of the area. Escutcheon narrow, depressed, 

 and lengthened ; its superior border is much raised : the area is narrow and flattened, 

 divided by an oblique mesial furrow, and bounded by two inconspicuous slightly knotted 

 carinas, which disappear altogether posteally ; near to the apex the area has a few 

 transverse plications, but the surface generally is nearly smooth. The lines of growth 

 are only faintly marked. The other portion of the shell has the first-formed six or seven 

 rows of costse entire and smooth, they pass obliquely downwards from the anterior 

 border, and curve to the carina at a right angle ; the subsequently formed costse consist 

 of two portions, the anteal series consist of a few narrow and depressed subnodulous 

 costae, which pass obliquely downwards to the middle of the valve, where their extremities 

 are contiguous to the extremities of a few, much larger, nodose varices which pass upwards 

 almost perpendicularly to the carina. 



Much variability in the costse is observable in different specimens, and not un- 

 frequently the ornamentation over the greater portion of the valve is effaced ; the later- 

 formed costae are usually disunited and very irregular. Om^ largest example has thir- 

 teen costae. The species was at first mistaken for T. Goldfussii, Ag. {Lyrodon literatum 

 Goldf.), which is more lengthened, and has a more prominent kind of ornamentation ; it 

 occurs in the Upper Oolites, but has not been discovered in Britain. 



The smallest of our figures exemplifies a young shell with the costae prominent, 

 smooth, and ridge-like ; they are united to the carina, where they form a projecting 

 angle, and pass undivided across the area as somewhat smaller costellse. Some doubt 

 may exist whether the minute figure of T. cuspidata, Sow., given in the ' Mineral Con- 

 chology,' pi. cvii, figs. 4, 5, is intended for a dwarfed example of this form, or for the 

 young state of T. Moretoni, as there is no very clear distinction between them ; but as 

 the first-formed costae of T. Moretoni are usually smaller and more closely arranged than 



