26 BRITISH FOSSIL TRIGONE. 



(T. Rigauxiana, Mun.-Ch. 1 ) ; in the latter species the comparatively narrow area and the 

 long oblique slope of the hinge-border are essentially different, and also the absence of 

 the wide, smooth, ante-carinal space. Another allied species from the same formation in 

 North -Western Germany is T. verrucosa, Credner, 2 but the latter is more erect, its 

 convexity is much greater ; the rows of tubercles are much more concentric, smaller, and 

 more numerous ; they become very small and attenuated as they approach to the position 

 of the carina ; this latter is also apparently destitute of tubercles ; it is, therefore, 

 clearly distinct. A clavellated species still shorter has also been figured in the same work 

 under the name of T. clivosa ; it has the rows of costae almost horizontal or sub-con- 

 centric, and appears to be destitute both of the smooth ante-carinal space and of tubercles 

 upon the marginal carina ; it is, therefore, more remotely allied to our species. 



Some specimens obtained from the same bed in Lincolnshire, and at the same locality, are 

 more gibbose, with more numerous rows of costae, each of which has smaller and more 

 numerous tubercles ; the posteal extremities of the costee curve upwards, and form small 

 faintly defined varices upon the smooth ante-carinal space ; these are exemplified by 

 Plate IV, figs. 6, and 7, which afford a marked contrast to fig. 5. In both varieties 

 the lines of growth are strongly defined over the entire surface. As the test has undergone 

 considerable change, no portion of the granulated tegument remains. 



Stratigraphical position and locality. The Kimmeridge Clay of Market Rasen, 

 Lincolnshire. The name is intended as a slight recognition of services to Jurassic 

 Geology rendered by Mr. John W. Judd in Lincolnshire and the adjacent counties, 

 during his labours as an officer of the National Geological Survey. 



Trigonia triquetra, Seeb. Plate VI, figs. 1, a, b, 2. 



Trigonia triquetra, Seebach. Der Hannoversche Jura, 1864, p. 117, pi. ii, fig. 5. 



Shell sub-trigonal, depressed ; umbones elevated, pointed, and slightly recurved ; 

 anterior side very short, its border abruptly truncated ; lower and posterior borders 

 slightly curved, giving to the general form, with its pointed posterior and apical extremities, 

 an unusual trigonal appearance. 



The escutcheon is large, slightly depressed, flattened ; its length is equal to half of 

 that of the marginal carina and to more than twice its breadth ; the area is narrow, 

 slightly convex ; the posteal half is more depresssd than the other ; the sides of the valves 

 are flattened ; they have rows of large varices, or in other instances tubercles, which pass 



1 " Note sur quelques especes nouvelles du genre Trigonia," par M. Munier-Chalmas, ' Bui!, de la Societe 

 Linneenne du Normandie,' vol. ix, 18G3-4, (Caen, 1865), pi. iv, fig. 2. 



2 ' Ueber die Gliederung der obern Juraformation, &c.,' Heinrich Credner, Prag, 1863, pi. viii, figs. 

 23, a, b, c. 



