UNDULATE. 79 



formed costse in our species have also much greater irregularity than is seen in T. 

 Moretoni. Upon the whole the enlargement of the posteal portions of the costae in the 

 latter species appears to be the most reliable distinctive feature, as it is always present. 

 The remarkable variabihty above described will account for our having failed to identify 

 with the second figure of Agassiz the very imperfect examples from the Forest Marble of 

 Farleigh ; and it has only been after comparison with examples from several localities 

 that I have become convinced of the necessity of merging T. arata in T. undidata. By this 

 latter species is intended only the specimen npon table x of the work of Agassiz, 

 excluding table 6, figure 1, which appears to be a different species with a few large, 

 widely-separated varices upon the angle of the valves or marginal carina. 



In no British specimen examined, is the marginal carina and its row of tubercles so 

 large, as in the figure of Agassiz, table x, which appears to constitute the extreme limit of 

 its variability in one direction; other examples with the marginal carina nearly plain 

 constituting the opposite limit of variability. 



Stratitjraphical position and Localities. T. widulata has been obtained only in the 

 upper subdivision of the Great Oolite. Mr. Walton procured specimens in the Forest 

 Marble of Farleigh ; Mr. Cunnington in the Cornbrash of Hilperton, near Trowbridge. 

 The officers of the National Geological Survey have also obtained it in Northamptonshire 

 and Southern Lincolnshire ; our figured examples are from Edenham, near Bourne, 

 Lincolnshire. The specimen figured by Agassiz was from Piedmont. 



Trigonia Shaupiana, Lycett, sp. nov. PI. XV, fig. 11 ; PI. XVI, figs. 3, 4, 5, 6. 



Morton. Natural History of Northamptonshire, 1712, tab. vi, 



fig. 9. 



Shell ovately trigonal, convex ; umbones elevated, and slightly recurved ; anterior 

 side short, its border curved elliptically with the lower border ; hinge-border short, 

 nearly horizontal, terminating posteally in the wide, rounded, and produced posteal 

 border of the area. Escutcheon depressed, wide, and short ; its upper border is some- 

 what raised. Area very wide, flattened, but occasionally with some convexity, bipartite, 

 and bounded by two regular, small, delicately, and closely tuberculated carinae ; it is 

 traversed transversely by narrow, sparingly arranged, regular costellee, which become 

 evanescent posteally in specimens of advanced growth ; each costella has for its carinal 

 termination one of the small carinal varices ; the area occupies fully one third the surface 

 of the valve. The other portion of the surface has numerous (16 — 17) rows of minutely 

 tuberculated costse ; the first-formed four or five rows are nearly concentric, narrow, 

 elevated, and only slightly tuberculated ; the succeeding rows are closely arranged 

 posteally ; they descend from the carina almost perpendicularly to the middle of the 



