36 E. A. WALFOKD ON THE STKATIGEAPHTCAL 



Shell ovately trigonal, moderately convex, umbones erect, pointed 

 and antero-mesial. The specimen (fig. 5, PI. I.) measures in height 

 11 inch, breadth 1§ inch, depth through both valves -i-| inch. An- 

 terior side produced and curving somewhat steeply to meet the lower 

 border ; superior border slightly convex, making but a faint angle 

 with the lower-area border. Inferior margin of the shell rather 

 straightened. Escutcheon concave and lengthened. Area convex, 

 occupying about a quarter of the surface of the shell. Marginal 

 carina depressed and not prominent ; mesial line of the area indi- 

 stinct ; area covered with fine transverse lines. The nearly perpen- 

 dicular costae, about ten in number, proceed from the . marginal 

 carina downwards, beginning near to the umbones ; they are boldly 

 marked until they reach the transverse subtuberculate costse pro- 

 ceeding from the anterior margin. The transverse costse dip gently 

 downwards from the anterior border. Traces of transverse striae 

 cover the better-preserved portion of the surface. 



The example described is much less convex than T. literata, and 

 in the ornamentation the perpendicular costae are not prominent 

 over more than a third of the surface, whereas in T. Uterata that 

 series occupies its larger portion. 



A second group of shells (figs. 4, 6, 7, PI. I.) presents many 

 points of divergence from the form just described. In some of 

 them, as in figs. 6 & 7, the outline is more oblique, and the anteal 

 costse are either reduced to insignificant proportions, or are appa- 

 rently absent. This condition much resembles the type of shell 

 figured by Lycett (pi. xiv. fig. 4, Brit. Foss. Trigonise), who speaks 

 also of the very variable ornamentation of T. Uterata. The per- 

 pendicular costse of this variety of Trigonia northamjotonensis enlarge 

 as they pass downwards from the marginal carina, and cross towards 

 the anterior or lower border. 



Eig. 4 presents an intermediate stage between the forms repre- 

 sented by figs. 5 & 6. The anterior tuberculate costse are seen 

 curving slightly downwards, transversely, towards the posteal series, 

 the space between the two series being occupied by a few scattered 

 tubercles. 



Trigonia navis, Lamk., has a more trigonal contour, and a nearly 

 vertical anterior border. The Northamptonshire shells appear to be 

 local representatives of the group Scaphoidse, finding a place some- 

 where between T. navis, Lamk., on the one hand, and T. Uterata, 

 T. & B., on the other. Their state of preservation is far from 

 satisfactory : the soft test has peeled off more or less by adhesion 

 to the tenacious blue clays in which the shells are imbedded, and 

 this has in nearly every case destroyed the beauty and prominence 

 of the ornamentation. Perhaps the finding of better examples 

 may ultimately allow the two varieties figured to be recognized as 

 species. 



I am indebted for my specimens to Mr. B. Thompson, E.G.S., of 

 ISTorthampton, who has been describing the Lias deposits of the 

 county. The Trigonice were obtained from Moulton, near North- 

 ampton, and rough casts have also been found at Bugbrook in the 



